perf: fix top 5 high-performance-go violations (reflect-driven sorts + strconv)

[jeduden]

Summary

Scheduled performance audit against
docs/development/high-performance-go.md.
Three background agents scanned internal/rules/, the rest of internal/, and
pkg/+cmd/ for allocation-budget violations, "Patterns to avoid"
anti-patterns, and struct-layout/reflect issues. The codebase turned out to
already be heavily scrubbed for these patterns (most candidates the agents
flagged were already-correct, documented optimizations, or — in one case
(internal/config.resolveEffectiveKinds) — a map allocation that Go's escape
analysis already proves never reaches the heap, verified with
go build -gcflags="-m=2").

Two candidate fixes were caught and dropped during review. A first pass
picked pre-sizing two allocation-heavy maps/slices
(internal/lint.CollectCodeBlockLines/CollectPIBlockLines, and the
diagnostics slice in MDS006/007/008) as part of the top 5, backed by
allocation-count tests on synthetic worst-case fixtures. The xhigh-severity
/code-review pass (round 1) empirically wall-clock-benchmarked both against
origin/main on realistic input rather than trusting the alloc-only tests,
and found both were real regressions on the common case — a two-pass
count-then-build costs ~80% more CPU than a single append pass when there are
few or zero violations (the typical file), for zero allocation benefit, since
growing a nil slice to a small result already costs one allocation either way;
similarly, an eager map-size estimate walk adds a full extra AST traversal for
zero benefit on the common code/PI-block-free file. Both were reverted and
replaced with two other real, safely-fixable violations, keeping the set at
five:

1. pkg/mdsmith/workspace.go (buildDirIndex) — sorted each directory's
   []fs.DirEntry with sort.Slice, which drives reflect.Swapper
   internally (the "reflect in hot paths" anti-pattern), already fixed the
   same way elsewhere in this codebase. Runs once per directory on every
   NewMemWorkspace call — the WASM/Obsidian/embedded-host path. Extracted
   into sortDirEntries using slices.SortFunc.
2. MDS061 (listmarkerspace, default-on) — itemVerdict built its
   diagnostic message with fmt.Sprintf and two %d verbs, driving fmt's
   reflection-based formatting path, unlike every sibling structural rule
   (listindent, headingincrement, atxheadingwhitespace,
   noduplicateheadings), which uses strconv.Itoa + string concatenation.
   Only builds on a confirmed violation, so no double-pass tradeoff.
3. cmd/mdsmith CLI output sorts (rename.go, deps.go, backlinks.go,
   buildpass.go) — five sort.Slice/sort.SliceStable call sites sorting
   command-output result lists, same reflect-driven anti-pattern as Add heading-max, code-block-max, and stern mode to line-length rule #1.
   Extracted each into a named sortXxx helper using slices.SortFunc /
   slices.SortStableFunc (preserving stability where the original used
   SliceStable).
4. pkg/goldmark/util/util.go (PrioritizedSlice.Sort) — same
   reflect-driven anti-pattern; this one method orders every default block
   parser, inline parser, paragraph/AST transformer, and node renderer
   goldmark registers, so fixing it once covers five call sites across the
   parse/render pipeline (pool-bounded, not per-file, but on every worker's
   pool-fill path).
5. internal/fix sorts (fix.go) — four more sort.Slice call sites
   (fixable rules by ID ×2, deduplicated diagnostics, per-file rule fix
   counts), same anti-pattern, run once per mdsmith fix invocation or cache
   build.

Every fix in this final set is a pure comparator-mechanism swap (single pass,
no added traversal), so none carries the common-case-regression risk that
sank the two dropped candidates. Each carries a dedicated allocation test
(0 allocs/op after, 3–7 before) that was verified red against the pre-fix
code before the fix made it green.

Code review: three xhigh-severity /code-review passes, as required.
Round 1 found the two regressions described above; they were reverted and
replaced, and the corrected set was re-validated (full suite, twice, plus
golangci-lint and mdsmith check). Rounds 2 and 3, run against the
corrected set, both came back clean (no findings) — including a -race
build/test pass and a stability/comparator-direction re-trace of every
extracted sort helper in round 3.

Test plan

• go build ./..., go build -race ./...
• go vet ./..., gofmt -l clean
• go test ./... (full suite green, twice in a row, plus -short and
  -race on touched packages)
• go tool -modfile=tools/go.mod golangci-lint run (0 issues, repo-wide)
• go run ./cmd/mdsmith check . (584 files, 0 failures)
• Each fix's test verified red before green
• Both dropped candidates were wall-clock-benchmarked against origin/main
  to confirm the regression before reverting (not just alloc-count tests)
• Three xhigh-severity /code-review passes — round 1 found and fixed 2
  regressions (described above); rounds 2 and 3 clean

🤖 Generated with Claude Code

https://claude.ai/code/session_01MTRxsATsWYv5rHeBsuCZuj

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.78%. Comparing base (3aa5a39) to head (6c87bf2).

Additional details and impacted files

Components	Coverage Δ
Go	98.77% <100.00%> (+<0.01%)	⬆️
TypeScript	99.54% <ø> (ø)

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[jeduden]

Review pass complete — no changes needed.

• Merged main into this branch (no conflicts; main only gained a new plan doc unrelated to this diff).
• Ran /code-review at high severity against the PR's actual diff (6b29d9e...a1a0f50): no findings. Verified every extracted comparator preserves original sort-key order and stability (SliceStable→SortStableFunc, Slice→SortFunc, never crossed), every dropped "sort" import has no remaining sort.* call, and the reconstructed listmarkerspace message is byte-identical to the original fmt.Sprintf output.
• Skipped /mdsmith-security-review — this diff is a pure stdlib sort/format refactor with no untrusted-input parsing, path resolution, or recipe execution touched.
• Validated on the merged head: go build ./..., go vet ./..., go test ./... (full suite green), go tool -modfile=tools/go.mod golangci-lint run (0 issues), go run ./cmd/mdsmith check . (585 files, 0 failures).

Pushed the merge commit (303ddc0) to bring the branch current with main; no other changes.

---

Generated by Claude Code

[jeduden]

Security review — no findings

Reviewed against the mdsmith threat model (PR/diff mode). This is a behavior-preserving performance change (sort.Slice/sort.SliceStable → slices.SortFunc, fmt.Sprintf → strconv) plus allocation-gate tests.

• No trust boundary touched: no recipe/command execution, path resolution, include/catalog, LSP, or Git-integration code (§0, §1, §7 unaffected).
• sortEditsByCharacterDesc in cmd/mdsmith/rename.go preserves the original comparator exactly — descending by Range.Start.Character, stable (cmp.Compare(b, a) matches the old > predicate) — so rename's in-place edit-offset math is unchanged (§2 in-place-rewrite safety intact).
• sortDiagnostics/sortRuleFixCounts/sortFixableRulesByID keep the same sort keys as the code they replace.

No §0 defense regressed. Signed off from a security standpoint.

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Generated by Claude Code

[Copilot]

🟢 Approval recommended

The changes are localized comparator/mechanism swaps with targeted allocation-gate tests, and no correctness issues were identified in review.

Pull request overview

This PR removes a handful of “reflect in hot paths” and fmt.Sprintf-driven formatting patterns by swapping sort.Slice/sort.SliceStable for slices.SortFunc/slices.SortStableFunc and fmt.Sprintf for strconv.Itoa + concatenation, and adds allocation-gate tests to lock in the wins.

Changes:

• Replace sort.Slice/sort.SliceStable call sites with slices.SortFunc / slices.SortStableFunc + cmp.Compare/cmp.Or in workspace indexing, CLI output ordering, fix output ordering, backlinks ordering, and goldmark’s PrioritizedSlice.
• Replace fmt.Sprintf diagnostic message formatting in listmarkerspace with strconv.Itoa + string concatenation.
• Add alloc-focused tests (skipping under -short and -race) plus per-package raceEnabled build-tag sentinels where needed.

File summaries

File	Description
pkg/mdsmith/workspace.go	Extracts sortDirEntries and swaps to slices.SortFunc + cmp.Compare for directory entry ordering.
pkg/mdsmith/sortdirentries_alloc_test.go	Adds an allocation-gate test ensuring the directory entry sort avoids reflect-driven sorting.
pkg/goldmark/util/util.go	Updates PrioritizedSlice.Sort to use slices.SortFunc + cmp.Compare.
pkg/goldmark/util/race_on_test.go	Adds raceEnabled sentinel for alloc-test skipping under -race.
pkg/goldmark/util/race_off_test.go	Adds non-race raceEnabled sentinel variant.
pkg/goldmark/util/prioritizedslice_alloc_test.go	Adds alloc-gate test for PrioritizedSlice.Sort.
internal/rules/listmarkerspace/rule.go	Replaces fmt.Sprintf with strconv.Itoa + concatenation for the diagnostic message.
internal/rules/listmarkerspace/race_on_test.go	Adds raceEnabled sentinel for alloc-test skipping under -race.
internal/rules/listmarkerspace/race_off_test.go	Adds non-race raceEnabled sentinel variant.
internal/rules/listmarkerspace/alloc_test.go	Adds alloc-gate test pinning itemVerdict message-build allocation behavior.
internal/fix/fix.go	Replaces several sort.Slice call sites with helper functions using slices.SortFunc.
internal/fix/sortnoreflect_test.go	Adds alloc-gate tests for the new sort helpers in internal/fix.
internal/fix/race_on_test.go	Adds raceEnabled sentinel for alloc-test skipping under -race.
internal/fix/race_off_test.go	Adds non-race raceEnabled sentinel variant.
internal/backlinks/backlinks.go	Replaces sort.SliceStable with slices.SortStableFunc via sortBacklinkRecords.
internal/backlinks/sortnoreflect_test.go	Adds alloc-gate test for backlink record sorting.
internal/backlinks/race_on_test.go	Adds raceEnabled sentinel for alloc-test skipping under -race.
internal/backlinks/race_off_test.go	Adds non-race raceEnabled sentinel variant.
cmd/mdsmith/rename.go	Replaces reflect-driven stable sort with slices.SortStableFunc for edit ordering.
cmd/mdsmith/deps.go	Replaces reflect-driven stable sort with slices.SortStableFunc for deps output records.
cmd/mdsmith/buildpass.go	Replaces reflect-driven stable sort with slices.SortStableFunc for build target output.
cmd/mdsmith/sortnoreflect_test.go	Adds alloc-gate tests covering the CLI sort helpers.

Review details

• Files reviewed: 22/22 changed files
• Comments generated: 1
• Review effort level: Lite

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[jeduden]

🟢 Merge Queue — picked up

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[jeduden]

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[jeduden]

✅ Merge Queue — merged

This PR landed on main via commit b48e90c. CI run that validated the merge.

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