refactor(thumbnail): [PKG-TOOL-THUMB-04] preserve approved published defaults

packages/artplayer-tool-thumbnail/README.md

@@ -4,9 +4,30 @@ Thumbnail tool for ArtPlayer
 
 Implementation and migration notes: [ARCHITECTURE.md](./ARCHITECTURE.md).
 
+The default constructor preserves published 3.5.31 behavior: fixed `height: 90`,
+`delay: 300` before the video event and each frame, and twice that delay before
+completion. Numeric height/delay values are clamped to 10-1000. File selection
+retains the input value. Existing methods, including `creatCanvas` and
+`creatScreenshotDate`, retain their names.
+
+Consumers of the unpublished 4.4 workspace behavior should select it explicitly:
+
+```js
+const tool = new ArtplayerToolThumbnail({
+  fileInput: document.querySelector('input[type=file]'),
+  compatibility: 'workspace-4.4',
+})
+```
+
+This mode calculates height from the video aspect ratio, emits `video` synchronously,
+has no fixed extraction delays and clears the file input value. Class-level
+`DEFAULTS` always describes the published policy, including `delay`; it does not
+vary by instance. Both modes wait for frame readiness and cancel owned work on
+source replacement or destruction.
+
 ## Demo
 
-[https://artplayer.org](https://artplayer.org/?libs=./uncompiled/artplayer-tool-thumbnail/index.js&example=thumbnail)
+[https://artplayer.org](https://artplayer.org/?libs=./uncompiled/artplayer-tool-thumbnail/index.js&example=tool.thumbnail)
 
 ## License