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fix(chat): preserve reading position across history prepends (#65)

* fix(chat): preserve reading position across history prepends

"History prepend with anchor preservation" is a headline behavior that
did not work: scrolled away from the bottom, a prepend of older messages
teleported the viewport into the prepended region and evicted the message
being read. The component's capture-mutate-restore anchor protocol was
wired only to ResizeObserver measurement callbacks; a messages-array
prepend arrives via prop reactivity, where no anchor was ever captured.

Detect a prepend in `$effect.pre` (old ids reappearing shifted by
newN-oldN, endpoints sampled — mirrors ChatHeightCache.sync's fast path)
and, when the user is scrolled away, capture the reading anchor from the
still-pre-paint DOM. Restore it via the existing VisualAnchor machinery,
deferred to `tick()` so the corrected scrollTop is written only after the
taller content exists (an earlier write would clamp to the old
scrollHeight). For the onNeedHistory-mid-scroll entry point the restore
is not gated on canPreserveAnchor — the scroll-progress preserver is a
no-op for growth above the viewport — and the preserver's baseline is
re-fed via its public commitAdjustment so the two cooperate.

Following-bottom behavior is untouched. Adds red-first regression tests
pinning the reading message across both manual and auto-load prepends.

Fixes #64

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* fix(chat): pin the bottom through history prepends while following

A prepend while following bottom left scrollTop uncorrected — the
viewport fell off the bottom by the prepended height (~800px estimated
plus re-measure growth, 1123px peak) — and the count-growth effect then
smooth-animated the recovery, a visible up-then-down jump. Detect the
prepend in the existing $effect.pre site and re-pin instantly in the
same flush/tick window the scrolled-away restore uses; a prepend flag
suppresses the smooth snap... (continued)

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