Mara: "New scroll down behavior works, but jumps instead of scroll."
The autoscroll path in `afterAppend` + the MutationObserver re-snap
+ the tail-pill click handler all set `log.scrollTop = log.scrollHeight`
— instant jump. Reads as jerky on long mutations where the row
height grew a lot between the initial append and the body fill.
Switch to `log.scrollTo({ top: log.scrollHeight, behavior: 'smooth' })`
via a new `snapToBottom()` helper. All three call sites (afterAppend,
MO callback, pill click) route through it.
## Smooth-scroll vs `stickToBottom` flicker
`behavior: 'smooth'` fires a stream of scroll events as the position
eases toward the target. Without guarding, the scroll handler reads
the intermediate position, fails `isNearBottom()`, flips
`stickToBottom` to false — the next MO callback then skips the snap
and strands the operator mid-scroll.
Add a `smoothScrollingUntil` timestamp gate: every `snapToBottom()`
call (without `immediate`) re-arms it to `now + 800ms` (~Chromium /
Firefox smooth-scroll animation duration + headroom). The scroll
handler ignores events while the gate is active. Bursts of writes
coalesce into one smooth ride to the latest bottom rather than a
sequence of half-cancelled animations.
## Backfill replay
`currentNoAnim` is true during history backfill — the operator never
sees the intermediate positions there, so smooth scroll is just
wasted animation. `snapToBottom(immediate=true)` (and the noAnim
branch in afterAppend) falls back to instant scroll for that path.
Same shared `@hive/shared/terminal.js` is used by the dashboard +
per-agent terminal + flow page; all three inherit the change.
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