perf(ui): stop the transcript from moving after a session switch - #4105

chat-scroll-anchor captured a turn id plus its offset before content
landed above the reader and restored that position a frame later. This
is what browser scroll anchoring already does, and does better: the
browser compensates during layout, not a frame after it.

Measured directly: inserting 1500px above the viewport mid-scroll moves
the visible content 0px. Measured in place: with the arrival prefetch
gone, removing the hand-rolled anchoring leaves mean CLS unchanged
(0.085 to 0.090, byte-identical on three of four sessions). The
anchoring was carrying no load once nothing inserted content the reader
had not asked for, which is why it could only go after the prefetch.

Removing it also removes the double compensation it sat behind — a
scrollTop adjustment by the scrollHeight delta, applied whenever the
restore reported failure — and the pendingAnchor round trip through the
virtualizer's window installs and resize observer.

Added in its place: overflow-anchor stated on the transcript column so
the dependency is legible rather than inherited from the default, and a
one-pixel nudge when the earlier-history request starts at the very top,
where anchoring is suppressed and would otherwise let the incoming turns
jump the reader.

Generated-by: Claude Code

chat-surface-layout states that Astryx owns scrolling and new-message
following. arrival-bottom-pin was a second implementation of exactly
that, added in apache#2239 because ChatLayout exposed only scrollContainerRef
and contentRef, so its controller could not be reached. apache#2923 opened
that seam for unlockAutoFollow and the pin was never revisited.

Reading Astryx's controller, it already covers what six review rounds
put into the pin: resize-synthetic scroll events are excluded by
comparing scrollHeight and offsetHeight, a horizontal wheel is excluded
by requiring deltaY < 0, and gestures are scoped by binding to the
scroller itself rather than by testing where the pointer was. Its
initial fill positions in one frame instead of springing from the top,
which is what the pin's clamp existed to produce.

The one gap was reachability again: on a conversation change the patch
called lock(), which re-enters through the spring because the hook's
initial-fill flag was consumed at mount. Asking for the instant jump
directly closes it, in the patch that was already there.

Removing the pin leaves two moves Astryx cannot see, both now going
through the context: navigating to a turn and loading earlier history
release auto-follow, and "return to latest" resumes it. The second
needed the other half of apache#2923's seam, so the patch also exposes
scrollToBottom. Both are additive context fields to upstream.

data-turn-window went with the pin it gated: its ready state existed to
release the pin, and the fonts.ready wait plus fifty markdown polls plus
double rAF existed to time that release. The two E2E tests that waited
on it wait for a mounted turn instead, which is what they were after.
latestNavigationNonce was left write-only and goes too.

arrival-bottom-pin.test.ts is replaced by a test of what Maka still
owns, the two release moments, rather than a test of Astryx's internals.

Capability given up: a wheel or touch over the dock while the transcript
is animating now releases following, where the pin discriminated by
gesture origin; and returning to the bottom re-locks following, where
the pin's release was permanent for that arrival.

Generated-by: Claude Code

Adversarial review found three more defects, all one mistake. The
mechanisms this branch deleted were continuous: the anchor restored a
captured position whenever the content landed, and the pin clamped the
bottom frame by frame for the whole arrival. Their replacements are
one-shot — Astryx's controller acts on the frame it is called, and the
browser anchors at the instant content is inserted. The call sites moved
across unchanged, and each one now fires before the DOM reaches the state
it assumes.

Loading earlier history nudged the scroller off zero when the request was
made, but anchoring is suppressed or not at the moment the turns land, a
whole IPC round trip later — and the reader coasting upward is back at
zero by then. The compensation moves to the frame after the turns are on
screen, which is also the first moment their height is known.

Return to latest jumped in a promise callback that resolves before React
commits the newer range, so it jumped against the old geometry and
consumed the pending first fill, leaving the range that arrived after it
to spring down from the top. The jump is removed rather than repaired:
nothing ever required this button to arrive instantly, and the transcript
scrolling to the newest turn shows the reader what happened. With no host
calling it, scrollToBottom comes back out of the layout context.

Navigating to a turn released auto-follow on every transcript update, not
once per chosen target: the effect re-runs on messages so a target that
arrives before its turn still lands, and the release it used to make was
an idempotent no-op. Astryx's unlock is persistent, and the search target
is never cleared, so following stayed off for the rest of the session.

Also here, because this change is what surfaced them: the gesture
releases were gated on a spring being in flight, which never happens
under prefers-reduced-motion, leaving those readers unable to leave the
tail by wheel or touch — the predicate is following, not animating. And
overflow-anchor moves to the scroller that actually runs it; on the
content column it was inert. hasTurns had no reader left.

Generated-by: Claude Code