Second step of the virtualization plan. No virtualization yet. This PR
makes IntersectionObserver-based visibility tracking correct inside
scroll containers (sidebar, bottom-sheet), which is a prerequisite for
virtualizing the timeline.
- Introduces `TimelineViewport` — a grouped contract passed as a single
`viewport` prop on `ActivityTimeline`. Holds `scrollElementRef`,
`observerRoot`, `scrollMargin`, and `virtualizationEnabled`.
- `MessageRowWithObserver` and `LeadThoughtsGroupRow` now create their
`IntersectionObserver` with `root = observerRoot?.current ?? null`
instead of defaulting to the document viewport. Unread marking now
fires when rows enter their real scroll parent.
- `MessagesPanel` resolves the active scroll owner from `position`
(inline from parent ref, sidebar from `sidebarScrollRef`, bottom-sheet
from `bottomSheetScrollRef`) and passes it into ActivityTimeline.
- Tests: stubs `IntersectionObserver` to capture `options.root` and
asserts null when no viewport is passed, and the provided element when
`viewport.observerRoot` is set.
`scrollMargin` and `virtualizationEnabled` are included in the contract
but not consumed yet — they land in follow-up PRs (#4/#5).
Replace the per-item backward scan that located the most recent session
anchor with a single forward pass via useMemo.
Before: for every timeline item the render loop walked backward until
it found a lead-thought anchor, so N items produced up to N * N anchor
lookups on every render pass.
After: a single O(n) sweep builds previousSessionAnchorByIndex; render
time lookup is O(1). getItemSessionAnchorId is hoisted to module scope
so it is not recreated per render.
Behavior is unchanged. The three existing separator tests still pass,
and four new cases cover three-session transitions, long runs of
non-anchor items between thought groups, consecutive same-session
thoughts, and single-item lists.
