fix(namespace): serialize manifest mutations

[Xuanwo]

This fixes the directory namespace CI failure where single-instance concurrent create/drop operations on __manifest could time out with TooMuchWriteContention, especially in the Windows build.

Manifest mutations are now serialized within a single ManifestNamespace instance so concurrent operations stop racing on stale in-memory snapshots, and inline manifest maintenance now defers compaction/index merges until the table has accumulated enough fragments.

Context: https://github.com/lance-format/lance/actions/runs/24439767878/job/71401857043

[codecov]

Codecov Report

❌ Patch coverage is 72.72727% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance-namespace-impls/src/dir/manifest.rs	72.72%	3 Missing ⚠️

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