docs(site): [SITE-04] document chapter and ambilight contracts in both languages

refactor/changes/2026-09-16-SITE-04-chapter-ambilight-guides.md

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+# SITE-04 checkpoint: Chapter and Ambilight bilingual guides
+
+SITE-04 moves from todo to doing. This is planned documentation implementation,
+not one of the user-guided release reviews. Core's 963 declaration-member rows
+and the remaining ecosystem content still need semantic checks.
+
+Added four pages for Chapter and Ambilight, with local sidebar destinations and
+links to the existing online examples. Each Run Code block is copied unchanged
+from its actual `docs/assets/example` source. The shared version menu now states
+`6.0.0 (unreleased)` instead of the obsolete `5.3.x`. New HTML paths are recorded
+with `introducedAfter: dba354f8cd752bc2f82f5299997fb1ff60b553b5`; no future commit
+or publication is invented.
+
+The [content mapping](../site-content-review.md) records the source, declaration,
+README and architecture checks. Chapter documentation covers fields, finite
+duration, in-place sorting/gap insertion/Infinity replacement, synchronous
+update and error behavior, once-only metadata initialization, source changes,
+DOM hooks and cleanup. Ambilight covers all option defaults, ignored zIndex,
+sampling frequency and playback conditions, synchronous start/stop, readiness,
+pixel access, canvas proxies and the approved root/runtime type distinction.
+Both languages state the unpublished status and applicable validation limits.
+
+Visual inspection found empty Run Code elements rendered as tiny green blocks.
+Their existing hooks and code are preserved, with visible `▶ Run Code` text
+added to these four new pages and the four existing Audio/VTT pages. This is a
+label fix, not a change to the navigation handler or media implementation.
+
+## Verification
+
+- Node 24.21.0, Yarn 1.22.22 and TypeScript 5.9.3. Four exact TS snippets pass
+  strict NodeNext checks with `skipLibCheck:false` and `types:[]`.
+- Each new generated page has 29 distinct local link/fragment targets, all
+  checked against the generated files and actual IDs. All eight labeled pages
+  retain their exact original demo code and data-libs settings.
+- Desktop Chromium 153.0.8010.12, Firefox 155.0 and Windows WebKit 26.6 pass all
+  12 page cases: HTTP page load, sidebar destination, visible Run Code label,
+  exact code and libs forwarding, language link and no uncaught page errors.
+  Popups receive an isolated empty editor response: this verifies navigation,
+  not editor execution or media playback. No physical-device claim is made.
+- Two screenshot inspections show the initial missing label and the final
+  readable control/layout. The in-app browser also displayed the local Chinese
+  Chapter guide; its tool did not supply a version and no playback was observed.
+- Existing site-loading/documentation-pipeline tests: 17 pass, no failures or
+  skips, 1411.5968 ms. Targeted config lint, build:llm, build:docs, check:llm,
+  demo path checks and site inventory checks pass. Final build:docs took 8.96 s.
+
+Two initial static probes failed because the lightweight HTML parser preserved
+`className` casing while its CSS selector normalized that name. Direct attribute
+inspection corrected the probe. This did not require altering page markup to
+make the probe pass. The first full navigation run passed before the visual
+label finding; after fixing the labels the pages were rebuilt and all 12 cases
+were rerun. Original logs, reports and screenshots remain available in
+[the validation record](../baselines/site04-chapter-ambilight-guides.json).
+
+All generated HTML, site browser assets and LLM output came from the project
+build commands. Current inventory is 40 Markdown files, 49 HTML paths and 30
+examples; the original 29 examples and 36 HTML paths remain accounted for.
+The 21 library candidates retain valid build fingerprints; no library source,
+type, distribution, dependency or public API changed.
+
+SITE-04 remains doing, with 224 done / 22 doing / 39 todo across 285 tasks.
+Continue from the content mapping; SITE-05/EX-03 still own full page/demo
+acceptance and device requirements. Reverting this checkpoint removes the new
+guides/navigation and restores labels/generated outputs without touching library
+behavior. No formal review, push, remote dispatch, deployment or publication.
+
+The unstaged whitespace check did not include the four then-untracked generated
+HTML files. After staging, the full check reports eight whitespace-only template
+lines in those four files. Each location was inspected; the remaining staged
+files pass the check with exactly those four paths excluded. The full check is
+not reported as passing, and the generated HTML was not hand-edited to hide the
+template whitespace. The validation record retains the locations and exit codes.