refactor(async): remove fs_event module

[16bit-ykiko]

Motivation

fs_event is being removed outright (PR-2 of the async cleanup plan). The platform watcher backends proved hard to use and unreliable across OSes, and clice #473 already deprecated file watching in favor of periodic stat-based polling. Keeping the module meant carrying ~1500 lines of platform code plus ~2900 lines of flaky tests for an API with no remaining consumer.

Replacement: build a periodic poll with kota::timer + fs::stat (compare mtime/size). The README now carries a migration note saying exactly this.

Deleted

File	Lines
include/kota/async/io/fs_event.h	71
src/async/io/fs_event.cpp (all 3 platform backends)	965
tests/unit/async/io/fs_event_tests.cpp	525
tests/unit/async/io/fs_event_dir_tests.cpp	2322
tests/unit/async/fs_event_fixture.h (shared test fixture)	41

Plus:

• kota/async/async.h: dropped the fs_event.h include
• src/async/CMakeLists.txt: dropped the source entry and the Apple-only -framework CoreServices link — verified fs_event was its sole user
• xmake.lua: dropped the matching add_frameworks("CoreServices") block
• README.md: removed the fs_event mention, added the timer + fs::stat migration note (only fs_event-related README changes; other README staleness is out of scope)

Net diff: 8 insertions, 3936 deletions.

Explicitly preserved

• uv::poll_* wrappers — src/http/manager.cpp:239-327 uses them.
• The entire fs.h/fs.cpp API surface (stat / opendir / readdir / scandir / sendfile / …) — untouched.
• uv::latest_value_delivery in awaiter.h is dead code but not fs_event-owned; it stays for PR-7 (structural dedup) per the plan.

Testing

• cmake --build build clean.
• ./build/unit_tests --snapshot-dir=tests/snapshots: 1367 passed, 2 skipped, 0 failed (baseline ~1413 before removing the fs_event suites).
• Final grep -rn fs_event over include/ src/ tests/ examples/ and build files: zero residuals (only the intentional README migration note mentions the old name).

Summary by CodeRabbit

• Documentation

  • Updated the async runtime docs to reflect broader filesystem API coverage.
  • Clarified that filesystem change notifications are no longer available.
  • Added guidance for portable change detection using polling and metadata checks.

• Breaking Changes

  • Removed filesystem watcher support and its related examples/tests from the async runtime.

[coderabbitai]

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Review details

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 09a4437b-fa21-42f4-9f6e-7fa923e1598e

📥 Commits

Reviewing files that changed from the base of the PR and between 115f894 and 48ab565.

📒 Files selected for processing (1)

• README.md

📝 Walkthrough

Walkthrough

This PR removes the kota::fs_event filesystem watcher API entirely, including its header, implementation (Linux inotify, macOS FSEvents, Windows ReadDirectoryChangesW backends), and associated tests. Build files are updated to drop the source, dependency, and framework linkage, and README documentation is revised to describe the removal.

Changes

fs_event Removal

Layer / File(s)	Summary
Umbrella header and build wiring updates include/kota/async/async.h, src/async/CMakeLists.txt, xmake.lua	Removes the fs_event.h include from the async umbrella header, drops io/fs_event.cpp from CMake sources, removes macOS CoreServices framework linkage, and reorders an unrelated block in xmake.lua.
fs_event API, implementation, and tests removal include/kota/async/io/fs_event.h, src/async/io/fs_event.cpp, tests/unit/async/fs_event_fixture.h, tests/unit/async/io/fs_event_dir_tests.cpp, tests/unit/async/io/fs_event_tests.cpp	Deletes the kota::fs_event class declaration, cross-platform watcher implementation, test fixture helpers, and all directory/file watching test suites.
Documentation update for removed notifications README.md	Expands the async filesystem API description and documents that filesystem-change notifications are no longer provided, suggesting polling via kota::timer and fs::stat.

Estimated code review effort: 2 (Simple) | ~10 minutes

Possibly related PRs

• clice-io/kotatsu#125: Directly overlaps with the removed kota::fs_event public API, implementation, and tests.
• clice-io/kotatsu#157: Modifies the same fs_event implementation/lifecycle code paths that this PR deletes.
• clice-io/kotatsu#174: Touches the same fs_event_tests.cpp and fs_event_dir_tests.cpp files that this PR removes.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: removing the async fs_event module.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch refactor/remove-fs-event

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