[dashboard-language] Built-in pages: overview provenance and data-state render slice

Caution

Protected files were modified in this change.
This pull request is in request_review mode and requires explicit human scrutiny before merge.

Protected files: README.md

Milestone

Built-in pages

This increment adds a browser-rendered prototype slice for the overview built-in page.

Added pages/dashboard/src/presenter.js, a tiny built-in page presenter that renders:
- page headings,
- independent availability, completeness, and freshness summaries, and
- per-source provenance entries derived from declarative built-in definition.views and runtime source metadata.
Replaced the static smoke test with a Playwright browser test that renders an overview built-in page and verifies independent data-state text plus provenance output.
Added validator acceptance coverage showing that overview built-in definitions can conservatively carry source-metadata-bearing views for this slice.
Updated pages/dashboard/PLAN.md and pages/dashboard/README.md for the new built-in rendering increment.

Section 10 Built-in Pages
- DLS-PAGE-002
- DLS-PAGE-014
Section 12 Validation and Errors
- existing validator behavior exercised by the new acceptance fixture

npm install ✅
npm run typecheck ✅
npm run lint ✅
npm test ✅
npm run test:e2e ⚠️ blocked by infrastructure: Playwright Chromium executable is not provisioned in this environment (browserType.launch: Executable doesn't exist)

Browser end-to-end tests are authored and included, but local Playwright execution is blocked here because the Chromium binary is unavailable.

Built-in pages — render one additional Section 10 built-in page from declarative definitions rather than only validating its coverage.

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