Separate timeline head state from conversation context

[ymichael]

Human comments

What was wrong

Latest plan/goal snapshots were inserted into conversation context before its bounds were calculated. An old plan could pull thousands of unrelated turn markers into a small current page. Projecting that snapshot as a partial historical turn also made current row ordering depend on unrelated head state.

What changed

Load latest head state separately and use it only for plan/goal extraction. Conversation events determine context bounds, grouping, and pagination. Include head-state rows in selection-cache byte accounting.

Stacked on #3874; this PR contains only the head-state separation.

How you verified

• Added regressions for an old plan across 100 turns, auxiliary state preserving current row order, and late changes to omitted commands retaining the pagination merge guard.
• 406 thread-view tests, relevant timeline server tests, client-core tests, and server/thread-view/client-core typechecks passed. Final head-state and selection-memo run: 18 tests passed.
• Compared actual default cold, repeat-open, and unchanged-update routes across all 2,385 threads in a consistent read-only main-DB snapshot. Rendered rows and head state match throughout. 41 threads change only olderRowsSourceSeqEnd, because auxiliary historical snapshots no longer participate in conversation projection. One already-deleted thread returns the same 404.
• Cold Post-merge cop: 117.55 → 35.45 ms after the index fix, with 7,364 → 901 selected events. S10 remains at 7,161 events. Broad latency flags were rechecked over nine measured samples across 19 threads; none retained a >10% and >5 ms regression. Focused Post-merge cop: 109.22 → 36.06 ms. Measurements are server-route timings on a shared host, not browser rendering timings.

AGENT GENERATED