## Why
Evidence capture previously reloaded the full history without tracking consumer progress. Live consumers need to read new originals from activation onward and acknowledge them after downstream writes succeed.
## What changed
- Add an in-memory `EvidenceCursor` that starts at the current complete tail without backfilling, captures a fixed unread range, and supports partial acknowledgements. Reads leave progress unchanged.
- Add `ThreadStore::read_original_history` to read original records and their source positions through a fixed cutoff. Local storage supports append-only paginated root histories, including compressed rollouts, and defers unfinished records.
- Reject unsupported histories and invalid ranges, and retain source ordinals as evidence IDs.
## Testing
Add coverage for partial records, compressed suffixes, fixed cutoffs, mismatched thread positions, and truncation. Extend evidence tests to cover activation without backfill, partial acknowledgements, later appends, stale captures, and unchanged cursor progress after capture errors.
GitOrigin-RevId: c7a13749fedc4f44fadddc9dd9875043bccebd95
felixxia-oai
ac03f42e1a