perf(table): index snapshots by ID

[fallintoplace]

Summary

• Index snapshot IDs in table metadata and metadata builders.
• Keep the index in sync when snapshots are added or removed.
• Keep a safe fallback for package-level fixtures that replace the snapshot slice.
• Add regression coverage and a lookup benchmark.

Performance

Snapshot lookup is roughly 150x faster in the 10,000-snapshot benchmark on an Apple M1 Pro.

Approach	Result	Relative speed
Indexed lookup	49 ns/op	~150x faster
Linear scan	7.8 us/op	Baseline

This keeps the existing defensive copy behavior for returned snapshots.

Validation

• go test ./table
• go test -race ./table/...
• go vet ./table
• repository-wide tests for the remaining packages