test: guard document section nav against Production UI drift

[BigSimmo]

Summary

PR #1311 already fixed and merged the Production UI failures (duplicate document-overview id + obsolete document-viewer section nav in ui-smoke). This follow-up adds cheap regression guards so the same class of drift fails in verify:cheap / unit CI before Production UI.

RAG impact: no retrieval behaviour change — tests and search-chrome contract docs only.

Root cause (already fixed on main via #1311)

1. DocumentClinicalSummary and the DocumentViewer overview wrapper both claimed id="document-overview" → expectDomIntegrity failed.
2. ui-smoke still drove the retired in-flow “Document viewer sections” nav and required a “Tables and diagrams” sheet row for the lithium demo doc (image_count: 0), which omits that row from the section index.

Prevention in this PR

• tests/document-section-nav-contract.test.ts — source contracts for single overview-id owner, phone sheet selectors, lithium image_count: 0, and ui-smoke wiring.
• DOM assertion that DocumentClinicalSummary does not claim document-overview.
• Document the invariants in docs/search-chrome-behaviour.md §22.

Validation

• npm run test -- tests/document-section-nav-contract.test.ts tests/document-clinical-summary.dom.test.tsx tests/document-section-index.test.ts tests/header-scroll-hide-contract.test.ts → 41 passed
• Full Production UI not re-run here; product fix already green on chore: organize dirty work from claude/navigation-pane-mockups-0600af #1311 tip before merge.

Clinical governance preflight

• Not applicable: no ingestion, answer generation, search/ranking, source rendering, privacy, or clinical output behaviour changes.

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📥 Commits

Reviewing files that changed from the base of the PR and between 7581cfc and 42e6f83.

📒 Files selected for processing (3)

• docs/search-chrome-behaviour.md
• tests/document-clinical-summary.dom.test.tsx
• tests/document-section-nav-contract.test.ts

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