feat(dir): shard directory catalog manifest

[jackye1995]

Summary:

• add DirectoryNamespace manifest_shard_count support
• route manifest catalog objects across _manifest_shard* Lance tables
• keep existing single-__manifest behavior as the default
• cover sharded table operations and table-version storage with focused tests

Validation:

• cargo fmt --all
• cargo test -p lance-namespace-impls test_sharded_ -- --nocapture
• cargo check -p lance-namespace-impls --all-targets
• cargo clippy -p lance-namespace-impls --tests -- -D warnings
• git diff --cached --check