Let plugins reveal their own content in the thread timeline

[brsbl]

Human comments

What was wrong

Plugins could expand a section inside a reply, but DOM scrollIntoView fought the timeline's bottom anchoring. Digests' routine-email counts opened content above the visible area, with no SDK hook to release the anchor.

What changed

• Added experimental_useThreadScroll().scrollIntoView(element, { block, behavior }), restricted to the calling slot instance and containing timeline; requires Plugin SDK 0.6.18 or later.
• Preserve the requested position through animated expansion; resume anchoring on new timeline events or the user's return to bottom.
• Respect reduced motion and the sticky composer; include SDK types, Plugin Guide documentation, regression tests, and ::scroll-demo usage.

How you verified

• Remote final-head CI passed, including app and SDK tests, Linux/Windows suites, package smoke, type checks, lint, and SDK version guards.
• Chrome for Testing 151.0.7922.71: desktop and mobile expansion settled at the section heading after three seconds; no captured page errors.
• Verified hard reload, client navigation, wheel return to bottom, and reduced motion (instant calls despite requesting smooth).
• Code review of 4d15c1da0a..60288c11e2 covered SDK ownership and anchoring and identified composer overlap; fixed it and added end/nearest coverage. Tests also cover ownership, unmount, resize, animated expansion, and new-event resumption.

Before: 4d15c1da0a · After: a2798ae240 · 2× pixel density. Same synthetic thread, directive markup, route, and expanded state in the isolated branch web app. The baseline fixture uses DOM scrolling; the candidate uses the new SDK hook.

Expanded routine emails	Before	After
Desktop · 1440×900
Mobile · 390×844

BB-Thread-ID: thr_bfxsh3mzue

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[brsbl]

Closing: the user decided against new plugin APIs and core changes for this. Digests (brsbl/bb-plugins#240) fixes the scroll jump inside the plugin instead.