Tweak comments, benchmarking, types, and code.

[dlrice]

Small changes, a benchmarking convenience, and a doc tweak.

Fix

• mousemove no longer dispatches mouseover / mouseout on every pixel — only on actual enter/leave transitions. Adds 5 regression tests covering the transition cases.

Benchmark / dev server

• New web-dev-server.config.mjs resolves @nightingale-elements/* to each package's src/index.ts and transpiles TS on the fly, so dev/benchmarks/*.html runs without a yarn build.
• New yarn benchmark:variation-canvas script opens the benchmark page directly.
• New NightingaleVariationCanvas#whenDrawn() lets the benchmark await actual draw completion instead of guessing with double-RAF.

Polish

• MAX_HIT_RADIUS warns when a variant exceeds the hit-test cap.
• pointer-events: none on the background canvas (defense in depth).
• TODO on the extends withCanvas(NightingaleVariation) line capturing the migration cost when SVG is removed.
• Drop unnecessary as unknown as Parameters<typeof createEvent>["1"] casts — VariationDatum is structurally compatible.
• README: documents the getVariantEnd fallback as a second parity gap.

Type re-export tweak

• packages/nightingale-variation/src/index.ts uses export type { … } for the existing type re-exports. Same public API, but per-file transpilers (incl. the new dev server) now drop them at JS-emit time so the browser doesn't try to bind nonexistent runtime exports.