fix(server): shard context writers by instance

[dcfocus]

Summary

• propagate the stable server instance ID into context-store creation and lazy reopening so replicas own distinct MemWAL shards
• centralize server context options and document that instance identity applies to every server-managed store
• add a two-replica regression test that verifies cross-shard visibility and distinct physical WAL shards

Context

#216 added writer-owned sharding to the core ContextStore. The server still opened context stores without shard_id, causing every replica to use the shared default shard. This completes the remaining server-side portion of #204 without changing the request API.

Testing

• cargo test -p lance-context-server (63 passed)
• cargo fmt --all -- --check
• cargo clippy -p lance-context-server --all-targets -- -D warnings

Closes #204