TT1: TimeStream retention + spool compaction + wormhole density #244
Description
From docs/spec-time-streams-and-wormholes.md: define retention and compaction behavior for TimeStreams (backlogs/spools) and how they relate to wormholes/checkpoints and durability/WAL epochs.
Scope:
- Define retention periods and compaction rules for stream payloads (semantic envelopes vs raw bytes).
- Define how spools are represented on disk and how they interact with wormhole segments / checkpointing.
- Define minimal invariants for replay correctness when compaction occurs.
Acceptance:
- Spec section defines retention/compaction and how replays validate digests across compacted segments.