Extract pure watch rebuild planning from DomStack

[bcomnes]

Summary

Closes #297. Stacked on #305 after #303, rebased onto master after #311 and #294 landed.

• 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 output state updates in DomStack.
• Preserve the merged routing rules by combining every direct and imported consumer, including modules shared between browser entry points and server-side consumers.
• Recompute global data when one of its imports changes even if that file has no direct output consumers; the page worker continues to choose subscribers by comparing values.
• Preserve full-page retries after failures and the resolved layout-chain maps from successful builds.
• Leave map refreshes with successful page builds rather than adding a redundant planner-owned refresh flag.

Validation

• Full Node suite, TypeScript, ESLint, installed dependency checks, and Playwright passed after rebasing onto the merged layout/subscription work.
• Filesystem-watch tests used Chokidar polling because native watchers are exhausted on this host.
• Nine pure planner tests run without file watching, filesystem fixtures, or sleeps, covering settings, consumer unions, deduplication, snapshot immutability, generated owners, global-data-only imports, browser/server overlap, failure recovery, structural events, and bundle scopes.

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 34148153534

Coverage increased (+0.7%) to 95.178%

Details

• Coverage increased (+0.7%) from the base build.
• Patch coverage: 3 uncovered changes across 1 file (239 of 242 lines covered, 98.76%).
• No coverage regressions found.

Uncovered Changes

File	Changed	Covered	%
index.js	79	76	96.2%
Total (2 files)	242	239	98.76%

Coverage Regressions

No coverage regressions found.

---

Coverage Stats

Relevant Lines:	7320
Covered Lines:	7134
Line Coverage:	97.46%
Relevant Branches:	1930
Covered Branches:	1670
Branch Coverage:	86.53%
Branches in Coverage %:	Yes
Coverage Strength:	196.52 hits per line

---

💛 - Coveralls