Extract pure watch rebuild planning from DomStack

[bcomnes]

Summary

Closes #297.
Stacked on #305, after #303 and #294.

• Extract pure filesystem-event classification and rebuild planning into lib/watch-plan.js.
• Plans explicitly describe skips, full rebuilds, esbuild restarts, and source-page/template/generated-owner filters.
• Keep resource ownership, discovery, execution, logging, and successful dependency/output state updates in DomStack.
• Preserve Track output dependencies in watch builds #294's worker-side property/global-data invalidation and resolved-layout reports.
• Refresh maps after bundle-triggered page work only when that page build succeeds.

Validation

• Six pure planner tests passed without file watching, filesystem fixtures, or sleeps.
• Cover settings, dependency precedence, deduplication, snapshot immutability, generated owners, structural events, and all bundle scopes.
• Full npm test passed outside the restricted runner, including Track output dependencies in watch builds #294's granular watch/cleanup regressions, lifecycle coverage, JSX add/remove coverage, Playwright, lint, and TypeScript.

Scope

This is the first organization/testability step, not a wider one-shot/watch pipeline rewrite.
Stable watch filenames, disabled watch manifests, and existing conservative structural fallbacks remain unchanged.

[github-actions]

Coverage Report for CI Build 34011365530

Coverage increased (+0.4%) to 95.29%

Details

• Coverage increased (+0.4%) from the base build.
• Patch coverage: 3 uncovered changes across 1 file (240 of 243 lines covered, 98.77%).
• 2 coverage regressions across 1 file.

Uncovered Changes

File	Changed	Covered	%
index.js	79	76	96.2%
Total (2 files)	243	240	98.77%

Coverage Regressions

2 previously-covered lines in 1 file lost coverage.

File	Lines Losing Coverage	Coverage
lib/build-copy/index.js	2	92.21%

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Coverage Stats

Relevant Lines:	8059
Covered Lines:	7852
Line Coverage:	97.43%
Relevant Branches:	2153
Covered Branches:	1879
Branch Coverage:	87.27%
Branches in Coverage %:	Yes
Coverage Strength:	302.09 hits per line

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💛 - Coveralls