Await watch shutdown and clean up failed startup

[bcomnes]

Summary

Closes #295.
Stacked on #294 so its watch dependency tests remain part of validation.

• Await Chokidar closure and stop accepting new or queued rebuilds before shutdown drains the active build.
• Dispose every acquired copy watcher, esbuild context, and development server even if another cleanup step fails.
• Clean up partial watch startup, including browser/service-worker esbuild startup failures.
• Stop watchers before removing test fixtures.

Validation

• Full npm test passed outside the restricted runner: Node tests, Playwright, lint, installed-check, and TypeScript.
• Six focused lifecycle regressions passed, covering awaited closure, repeated cycles, startup failure/retry, cleanup failure/retry, partial esbuild startup, active/queued rebuild shutdown, and stopping from the initial-build callback.
• The local EMFILE failures reproduce in the restricted runner but not outside it; this PR does not claim the missing await caused them.

Merge order

Merge #294 first, then retarget this draft to master if GitHub does not do so automatically.
The conventions and watch-planner drafts follow this branch.

[coveralls]

Coverage Report for CI Build 34011495814

Coverage increased (+0.4%) to 94.828%

Details

• Coverage increased (+0.4%) from the base build.
• Patch coverage: 2 uncovered changes across 1 file (95 of 97 lines covered, 97.94%).
• No coverage regressions found.

Uncovered Changes

File	Changed	Covered	%
index.js	58	56	96.55%
Total (2 files)	97	95	97.94%

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	8099
Covered Lines:	7864
Line Coverage:	97.1%
Relevant Branches:	2091
Covered Branches:	1799
Branch Coverage:	86.04%
Branches in Coverage %:	Yes
Coverage Strength:	297.08 hits per line

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💛 - Coveralls