refactor(vtt-thumbnail): [PKG-VTT-THUMB-03] own pending requests and preview resources

packages/artplayer-plugin-vtt-thumbnail/ARCHITECTURE.md

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+# VTT thumbnail maintenance
+
+The factory remains `artplayerPluginVttThumbnail(option)`. Its registration function is
+asynchronous and resolves to `{ name: 'artplayerPluginVttThumbnail' }`. Existing public
+declarations still describe a synchronous result; task PKG-VTT-THUMB-04 owns that conflict
+and the strict TypeScript migration. Do not infer the return shape from those declarations.
+
+## Modules and ownership
+
+- `src/index.js` composes registration, captures the VTT URL before awaiting, reads style
+  afterwards, and installs the `vtt-thumbnail` control with its existing CSS class/index.
+- `src/lifetime.js` owns one registration's destroy listener, event listeners and cleanup
+  callbacks. Close is idempotent. Cleanup continues after a callback fails. An independent
+  cancellation Promise settles pending work even if fetch does not honor AbortSignal.
+- `src/request.js` owns a native AbortController when available and reads the response body.
+  Destroy cancels both phases; late rejections are observed. Failed HTTP responses reject
+  before parsing. The controller cleanup is released when the request ends.
+- `src/parseVtt.js` is pure parsing with the historical timestamp floor, first inclusive
+  match data, raw xywh strings and lexical URL joining. Its old malformed-input limitations
+  remain under PKG-VTT-THUMB-03; extracting this function is not a parser rewrite.
+- `src/preview.js` owns the mobile 500ms timer and creates the setBar callback. Timer
+  generations reject old callbacks. Every style write checks lifetime state, including
+  callbacks captured before destroy. Geometry, mobile/desktop edges and cue selection
+  retain the latest published behavior.
+- `src/getVttArray.js` retains the old source helper signature as a fetch/parse wrapper.
+  The player registration uses the owned request module instead. Distribution/deep-import
+  review belongs to PKG-VTT-THUMB-06.
+
+The registration owns only the control element passed to its mounted callback. Before
+removing it, cleanup compares the live `art.controls['vtt-thumbnail']` alias with that exact
+element. It must not remove a later replacement. Partial listener/control installation is
+rolled back if initialization throws, preserving the original rejection.
+
+## Compatibility and intentional fixes
+
+Option/style remain live at their historical read points. The return stays a Promise;
+destroy during loading now settles it with the normal name result, without installing UI.
+Existing core plugin managers can complete their own destroyed-instance handling. This
+does not introduce a public destroy method or a new registration result field.
+
+Registration still downloads VTT once. A video restart does not reload it or reinterpret
+later option.vtt changes. Empty cues remain valid and hide the preview. HTTP failures now
+reject explicitly; before this fix, a 404 with a parseable body incorrectly displayed cues.
+The historical failure tests retain the former outcome rather than claiming it was valid API.
+
+The old 1.0.x export objects, earlier `thumbnails` control name and incorrect declarations
+are separately frozen and remain compatibility work for tasks 04-06. Current runtime work
+does not claim those consumer migrations or the full package refactor are finished.
+
+## Verification and future changes
+
+Use Yarn 1.22.22 and the repository's pinned Node version:
+
+```sh
+yarn test:vtt-thumbnail
+yarn test:browser test/browser/vtt-thumbnail-lifecycle.spec.js
+yarn build artplayer-plugin-vtt-thumbnail
+```
+
+The unit helper can load actual main/legacy files with ARTPLAYER_VTT_THUMBNAIL_ARTIFACT or
+the frozen former implementation with ARTPLAYER_VTT_THUMBNAIL_BASELINE=1. Historical tests
+are distinct from candidate expectations. Browser tests use real published/candidate cores,
+native fetch/abort, mouse hover, sprite decoding and cleanup. Their scope is not full old
+core/device coverage, installed tarballs, mobile Safari or the online editor.
+
+Modify parsing in parseVtt, requests in request, layout/timers in preview, and ownership in
+lifetime. Preserve the normal/failure lifecycle tests alongside any parser improvements.
+See ../../refactor/baselines/vtt-thumbnail-contract.md and the task 03 change record for
+historical evidence and outstanding acceptance work.