feat(experimental): add stable luDF sorting and top-K

[ibgreen-openai]

Goals

Expose stable, GPU-resident numeric sorting and top-K selection through the optional luDF dataframe API while preserving source record batches, stable row identity, explicit null behavior, and caller-owned command submission.

Changes

• Add immutable LuDataFrame.sortBy, LuDataFrame.topK, and matching query methods for float32, sint32, and uint32 columns, with typed direction, null placement, NaN placement, and existing bitonic/radix algorithm controls.
• Compile two stable per-batch GPU sorting passes using full-width monotonic numeric keys followed by explicit selected/null/NaN classes; preserve duplicate stability, signed zeros, infinities, the complete unsigned domain, and encoder-ordered filter updates.
• Publish sorted stable source-row identifiers, coherent top-K selection masks, and per-batch GPU counts without packing source chunks, submitting commands, or reading rows back to the CPU.
• Reject scalar constants, unavailable nullable sidecars, invalid limits/source offsets, and padded or interleaved scalar buffers before GPU allocation.
• Cover nullable floating-point and signed/unsigned extrema, empty batches, derived columns, retained owned-source leases, explicit null/NaN ordering, bounded top-K, and repeated graph encoding in focused Node and real-WebGPU tests.

Verification

• nvm use: Node v22.22.1.
• yarn install: not rerun; reused the already verified local dependency tree because the existing enterprise alpha registry dependency returns HTTP 403.
• yarn lint fix: passed after final source changes.
• yarn build: passed after final formatting.
• yarn test: Node 606 passed/1 skipped; real headless Chromium/WebGPU 1,581 passed/25 skipped.
• Focused luDF Node suite: 7 files/63 tests; focused real-WebGPU sorting suite: 5 tests.
• yarn website:build: passed and validated 461 documentation pages.
• (cd website && yarn build): passed and validated 461 documentation pages.
• yarn examples:typecheck: passed across all 46 example workspaces.
• yarn bundle-size: passed all seven existing budgets.
• Built optional ESM and CommonJS luDF sorting exports were loaded successfully.

Risks and follow-up

• Sorting is explicitly per existing source batch; global materialization, nonnumeric ordering, and sorting constant columns are intentionally unsupported.
• Nullable rows and NaNs are kept as explicit independently ordered classes; deselected rows always remain outside the published count.
• Independent physical-alias and scaled-dispatch prerequisites are tracked separately.