Third step of the virtualization plan. Pure refactor — no UI change, no
virtualization yet. Prepares the timeline for row-level windowing.
- Introduces `TimelineRow`, a discriminated union of `session-separator`,
`lead-thought-group` (pinned and non-pinned), `compaction-divider`,
and `message-row`. Each row maps 1:1 to a single visual element.
- Adds a `renderRows` useMemo that walks `timelineItems` once and emits
atomic rows, hoisting session separators out of the Fragment bundle
that used to pair them with their owning item. This is the shape a
windowing layer needs: each row measurable and addressable
independently.
- Extracts a `renderTimelineRow(row)` helper that switches on `row.kind`
and returns the same JSX the previous inline render produced. Logic
per kind is identical — keys, memoization, collapse props, pinned
thought "live" semantics — so there is no visual diff.
- The render body collapses from two blocks (pinned + `.slice().map()`)
into a single `renderRows.map(renderTimelineRow)` call.
Follow-ups will virtualize `renderRows` with measured row heights and
tighten observer/animation wiring; pagination, collapse state, zebra
striping, and `groupTimelineItems` are untouched.
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