[dashboard-language] Document model and validation

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, package-lock.json, package.json

Milestone

Document model and validation

Sections 4 and 12
Requirements: DLS-DOC-001, DLS-DOC-002, DLS-DOC-003, DLS-DOC-004, DLS-DOC-005, DLS-DOC-006, DLS-DOC-007, DLS-DOC-008, DLS-DOC-009, DLS-SAFE-001, DLS-VAL-001

Added src/specification.js with canonical document vocabulary and error-code constants.
Added src/validator.js with a first validator slice for YAML parsing, single-document enforcement, root and dashboard shape checks, canonical identifier validation, page/view uniqueness checks, key vocabulary checks, page-kind checks, built-in page-name checks, and coded YAML-path error reporting.
Added unit coverage in test/unit/validator.test.js for structural acceptance and rejection cases tied to the implemented requirement IDs.
Updated PLAN.md to mark the milestone complete, record one specification ambiguity, and log this run's verification results.

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

Browser end-to-end tests are still blocked by missing Playwright browser binaries in the workflow environment.

Semantic model

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