docs(site): [SITE-04] document thumbnail extraction and subtitle selection

refactor/changes/2026-09-16-SITE-04-auto-multi-guides.md

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+# SITE-04 checkpoint: Auto Thumbnail and Multiple Subtitles guides
+
+Four new bilingual pages and local sidebar entries describe the current source
+and compatible declarations. Original auto.thumbnail and multiple.subtitles
+examples are embedded unchanged, with visible Run Code controls. New HTML routes
+record pre-change HEAD 94adb77fd677f34c171ed1812cbda9512dc5e168.
+
+Auto Thumbnail covers every runtime option, truthy defaults, ignored height,
+ten-column geometry, progressive encoding, sampling times, independently loaded
+media, phase deadlines, cancellation and object URL ownership. The registration
+Promise only installs subscriptions. Its root declaration stays historically
+synchronous, while /runtime supplies the accurate type. Windows WebKit's first
+frame issue remains explicitly unresolved.
+
+Multiple Subtitles covers track fields and downloads, ignored onParser,
+unknown-name errors, ordered selection, reset, shared escape/configuration,
+style hooks and cancellation. It distinguishes registration from asynchronous
+host subtitle loading, and the npm1.2 root type from the accurate runtime entry.
+The original selection menu remains application code, not an automatic plugin
+feature. Detailed member/source mapping is in
+[the content tracker](../site-content-review.md).
+
+## Verification
+
+- Node 24.21.0 / Yarn 1.22.22 / TypeScript 5.9.3. Four exact snippets pass strict
+  NodeNext checks with skipLibCheck:false and types:[], without diagnostics.
+- Each page validates 33 distinct generated local file/fragment targets. All four
+  Run Code examples match original files exactly.
+- Desktop Chromium 153.0.8010.12, Firefox 155.0 and Windows WebKit 26.6 pass all
+  12 page cases, without retries or uncaught page errors: HTTP load, sidebar,
+  visible control, exact code/libs forwarding and language navigation.
+  Popup editor responses are isolated, so this does not verify editor execution,
+  thumbnail pixels, native subtitle display or physical devices.
+- The Chromium Chinese Auto Thumbnail screenshot was inspected for layout and
+  visible controls. Full-page screenshots are retained for all 12 cases.
+- Existing site-loading/documentation-pipeline tests: 17 pass, no failures/skips,
+  1262.1348ms. Config lint, build:llm/check:llm, build:docs, site inventory and demo
+  path checks pass. VitePress reports 7.96 seconds; Yarn build:docs 9.65 seconds.
+- Current inventory: 48 Markdown (24 per language), 57 HTML, 30 examples,
+  963 core declaration members and 204 assets. Original routes remain accounted
+  for; enumeration does not establish semantic coverage of all core members.
+
+The Auto Thumbnail architecture document still said the package version was
+unchanged. It now distinguishes the initial migration from current next-major
+preparation. This changes its input fingerprint: the old candidate's build gate
+is correctly stale, and REL-02 must rebuild/revalidate it from the committed
+inputs. The other 20 library build gates remain accepted. No old evidence was
+relabelled, and no library runtime, public declaration or dependency changed.
+
+See [the evidence record](../baselines/site04-auto-multi-guides.json). All HTML,
+site assets and LLM output came from repository build scripts. SITE-04 remains
+doing, with 224 done / 22 doing / 39 todo. Full core/ecosystem content, SITE-05,
+EX-03, native/device checks and release gates remain. No user-guided formal
+review, push, remote dispatch, deployment or publication was started.
+
+Reverting this checkpoint restores the previous guides, navigation, generated
+outputs and architecture sentence. Release evidence must always match the
+resulting input fingerprint; a revert is not permission to relabel reports.
+
+Full staged whitespace checking exits 2 on eight whitespace-only template lines
+in the four new generated HTML files. Exact paths/lines and blank content were
+inspected; excluding only those files passes. Generated HTML remains intact,
+and the full check is not described as passing. See the validation record.