fix(build): debounce webpack watch mode to avoid mid-write reads (#78)

[MonaAghili]

Summary

• Fixes webpack watch mode intermittently reads sdkjs source mid-write, breaking the concatenated bundle and increasing build times #78: npm run watch:cell (and watch:word/watch:slide/watch:visio) could intermittently produce a truncated sdk-all.js (<1KB) with a Module parse failed error, immediately followed by a correct rebuild.
• Root cause: webpack's default watchOptions.aggregateTimeout is 20ms (confirmed in the installed webpack@5.108.4 source). Editors can flush a single save as multiple MODIFY/rename events, especially over a WSL2/9p filesystem boundary. If a rebuild starts inside that 20ms window, sdk-concat.cjs re-reads the file mid-write and concatenates a truncated fragment into the ~400+ file bundle, producing a syntax error at the tail of the generated module.
• Fix: set watchOptions.aggregateTimeout to 500 in chunkConfig() in build/webpack.sdk.factory.mjs, shared by all four SDK targets (word/cell/slide/visio) and both chunk variants (min/all). watchOptions is a documented no-op for one-shot builds, so npm run build / CI are unaffected.
• Added WATCH_AGGREGATE_TIMEOUT env override for tuning on a given machine/CI watcher, consistent with this file's other env-tunable knobs. Invalid input (non-numeric/negative) warns and falls back to the default instead of throwing, so a stray/typo'd env var can never break a plain build.

Why 500ms

25x webpack's default, comfortably absorbing a multi-event editor flush, while staying small relative to this pipeline's warm incremental rebuild time (~5-8s dev). It's a debounce on the rebuild trigger, not a read-time stability check — sdk-concat.cjs still does a plain readFile once the timeout fires, so this narrows the race window rather than eliminating it structurally. A write that's still landing after 500ms of silence (very slow disk, network mount) could theoretically still race; a full fix would require content-stability polling or retry-on-parse-failure in the loader, which is out of scope for this issue.

Testing

• npm test --prefix build: 45/45 passing, including new coverage for:
  • default aggregateTimeout (500) on both chunks, all targets share the one factory
  • WATCH_AGGREGATE_TIMEOUT override, including 0 (explicitly honored, not treated as unset)
  • invalid override input warns and falls back rather than throwing, verified via sdkConfig() itself (not just the parsing helper)
• Live repro: ran npm run watch:cell-equivalent, then rewrote a config-listed source file in two staggered chunks (5-10ms apart) to simulate a multi-event editor save. Before the fix this pattern reliably races; with it, exactly one rebuild fired and the resulting sdk-all-min.js was full-size and passed node --check.
• Full production build of the cell module confirmed unaffected: sdk-all.js emitted at expected size (~19.8 MiB minified).

Not in scope

• Structural (read-time) fix in sdk-concat.cjs — discussed and deliberately deferred; see comment in webpack.sdk.factory.mjs.
• WSL2/9p inotify events not firing at all (a separate, unrelated failure mode watchOptions.poll would address) — not what webpack watch mode intermittently reads sdkjs source mid-write, breaking the concatenated bundle and increasing build times #78 reports.

Close #78