Adds the core SeamManager struct (#159) that uses V4 Flash to produce
append-only <archived_context> XML blocks at 192K/384K/576K thresholds.
No messages are deleted — soft seams are navigational summaries that
preserve the V4 prefix cache.
- seam_manager.rs: Flash-driven soft seam production, recompaction,
and cycle briefing replacement
- config.rs: [context] table with L1/L2/L3/cycle thresholds,
verbatim window, seam model, and per-model overrides
- compaction.rs: pub exports for plan_compaction, KEEP_RECENT_MESSAGES,
and CompactionPlan fields so SeamManager can reuse pinning heuristics
- cycle_manager.rs: pub CYCLE_HANDOFF_TEMPLATE for Flash briefing use
- main.rs: mod seam_manager registration
All 1,570 tests pass. Engine wiring follows in a subsequent commit.
Hunter Bown
12b1ae42c4