Module append 

Append pipeline for TimeSeriesTable.

This module contains the core append implementation plus the public wrappers. It is responsible for:

• loading/deriving segment metadata and logical schema,
• enforcing v0.1 schema rules (adopt on first append, otherwise exact match),
• computing segment coverage, detecting overlaps, and writing coverage sidecars,
• optimistic commit to the transaction log and in-memory state update. Keep new append-time invariants here so the flow remains centralized.