perf(precompute): prune snapshots by generation

[zzylol]

Closes #594.

Eliminates the per-rotation closedKeys allocation and duplicate canonical-key construction:

• series entries retain their canonical key from admission through serialization;
• sliding merged entries preserve that identity too;
• SnapshotCache uses reusable generation marks for liveness/pruning;
• cache writes concurrent with close serialization mark themselves in the current generation and are not accidentally pruned.

Adds delta-rotation benchmarks at 1k/10k/100k cardinality. Core tests and race validation pass.

Stacked on #602.