fix: include namespace in API V3 physical routing

[disksing]

API V3 keyspaces with the same local ID in different namespaces currently produce identical physical routing keys. Encode the routing prefix as mode + 32-bit namespace + 24-bit keyspace, including carry across the complete prefix for exclusive range bounds. RPC keys remain logical, and API V2 routing is unchanged.

Add TransactionClient::with_keyspace_identity to derive scoped clients while sharing the PD connection and region cache. Bound the shared V3 timestamp-oracle cache to 64 least-recently-used entries. In-flight requests retain their oracle until the response completes; evicted streams terminate when their final owner is released. Pending queues allocate on demand.

Restore compatibility with legacy PD metadata that omits the proto3 scalar keyspace ID zero, while continuing to distinguish explicit V3 identities. This fixes the existing DEFAULT-keyspace failure in the raw and transaction integration jobs against PD v8.5.5.

Validation: 143 client unit tests, including legacy wire decoding and oracle eviction/in-flight lifetime cases; Rust 1.93 format and all-target clippy with warnings denied; CSE integration using namespaces 7 and 19 with the same keyspace ID, covering bidirectional transactions, alternating cached reads, unbounded forward/reverse scans, pessimistic locks, and independent expired-lock resolution. The preceding legacy-compatibility commit also passed both GitHub integration jobs; CI is rerunning for the final oracle-lifetime change.

These protocol changes require coordinated PD/CSE/client rollout to a newly initialized V3 cluster; existing four-byte V3 data is not migrated by this PR.

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