Distributed block-table serialization (kv_cache_serde)

[inureyes]

Context

Disaggregated / decode-node handoff must transfer paged block tables and pool blocks, not dense snapshots, once the unified store is the source of truth.

Tasks

• Extend src/distributed/kv_cache_serde/ to serialize PagedSequenceState block tables plus the referenced pool blocks (and Turbo sidecars) and restore them via PagedBlockPool::restore_sequence.
• Validate a prefill-node to decode-node handoff reconstructs identical KV and identical first-token logits.
• Confirm refcount and block ownership are correct after restore (no double-free, no leak).

Acceptance criteria

• A disaggregated run reproduces single-node output within tolerance using the paged block-transfer path.
• Block accounting after restore matches the originating node.

Dependencies

Blocked by Phase 4 (unification).

Part of #116