TT1: TimeStream retention + spool compaction + wormhole density

[flyingrobots]

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.