fix autoscroll issue due to stale scrollableMap

[cehan-Chloe]

Root cause:
Page1 has autoscroll enabled. scrollableMap was never cleared on view transition from page1 to page2. When navigating to page 2, page 1's registered element IDs remained in the map. Duringthe React transition window — before page 1 components unmount — useEffect fired with stale page 1 IDs, found those elements still in the DOM, and called scrollIntoViewWithOffset. A concurrent window.scroll(0,0) from the player lifecycle fired mid-animation, causing the smooth scroll polyfill to land at the wrong position

Debug observation:
during transition, window.scroll(0,0) is called to reset position but meanwhile the autoscroll useEffect traversed the stale scrollableMaps and triggered multiple scroll animations. only stops until react unmount the page1 components and node is null then no further scroll.

fix:
Clear scrollableMap when a view transition fires, so stale registrations can't trigger scrolls on the new page. Wired this up via an onMount callback from AutoScrollProvider to the plugin's transition hook.

alternative considered: add setTimeout to the auto-scroll useEffect to make sure scroll triggers after react render.

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[codecov]

Bundle Report

Changes will increase total bundle size by 1.22kB (0.02%) ⬆️. This is within the configured threshold ✅

Detailed changes

Bundle name	Size	Change
plugins/auto-scroll/react	19.67kB	1.22kB (6.64%) ⬆️

Affected Assets, Files, and Routes:

view changes for bundle: plugins/auto-scroll/react

Assets Changed:

Asset Name	Size Change	Total Size	Change (%)
cjs/index.cjs	420 bytes	7.99kB	5.55% ⚠️
index.legacy-esm.js	402 bytes	5.84kB	7.39% ⚠️
index.mjs	402 bytes	5.84kB	7.39% ⚠️

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (de05e81) to head (7cc4366).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@     Coverage Diff     @@
##   main   #815   +/-   ##
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