refactor(tui): persist group expansion and exact scroll anchors

[jlongster]

Summary

Refactored to the agreed exact-target model:

• Scroll anchors identify a PartRef or group ID, plus a screen offset. Whole non-assistant messages use a canonical UI body PartRef.
• Group IDs derive from the first descendant’s message ID + part ID + group kind + nesting level (top level is 0). No projection-owned identity reconciliation.
• Tabs retain each session’s anchor and group expansion state in memory, clearing them on tab close.
• Recursive group views render from saved expansion state, then register actual parts and group headers.
• No automatic expansion, reveal flag, reveal loop, depth-ranked candidates, or group-as-message fallback.
• Expansion/collapse saves state immediately and updates the anchor after layout.
• Restoration locates the exact mounted target and applies its offset; existing bottom-padding support preserves offsets in short transcripts.

Fetching and boundary completion are explicitly deferred to the next PR. A change to the first member changes the group ID; unavailable targets use the existing missing-anchor fallback.

Verification

• TUI typecheck passes
• Full TUI suite: 1368 passed, 4 skipped, 0 failed
• Final focused anchor/group/tab tests: 45 passed
• 201/201 baseline screenshot pairs match for the existing presentation (84 grouped, 84 ungrouped, 33 disclosure states)
• Tests cover composite PartRef keys, canonical message refs, deterministic group IDs, append/prepend behavior, replacement cleanup, nested expansion retention and exact target geometry
• Full-app tests cover no automatic expansion, closed-summary restoration, retained expanded state, offsets within later parts of a message, and short transcripts at 48/80/120-column coverage
• Drive explicitly verified no automatic expansion, manual expansion, tab switching and restored expanded/collapsed states with exact offset preservation

Updated implementation: b087728684. Local work log: /tmp/opencode/grouped-timeline-work-log.md. Final demo captures: /tmp/opencode/anchor-state-fixed/run-6c96b6e1-b9da-45fd-a3c0-a64f74ebc586/generation-0/ (copying to /mnt/mail currently hangs).