feat(dir): add metadata update APIs

[jackye1995]

Stacked on #7008.

Adds native DirectoryNamespace APIs to update table and namespace metadata stored in the __manifest JSON metadata column. Metadata remains JSON, but the API treats it as a string-to-string map, with description as the UI-facing long-description key.

The sharded manifest path updates only the target shard fragment for the changed table or namespace row.

S3 benchmark, c7i.48xlarge, 1000 shards, 1 writer, 100 measured write-update-table-metadata ops per point. Initial rows were bootstrapped with one 50-100 word description and 10 key-value tags; measured writes update the same metadata shape.

Initial rows	TPS	P50 ms	P99 ms	Errors
100	6.049	151.2	295.9	0
1,000	6.185	149.4	320.6	0
500	5.907	161.8	347.3	0
10,000	5.009	174.8	470.5	0
50,000	5.718	160.7	330.8	0
100,000	3.551	279.8	385.9	0
200,000	2.927	347.7	536.9	0
300,000	2.863	323.7	601.2	0
500,000	2.875	338.2	507.5	0
750,000	2.300	418.2	725.4	0
1,000,000	2.462	381.7	735.4	0

Raw artifacts are recorded locally under ~/ai/analysis/lance/jack-dir-metadata-api/update-table-namespace-metadata-api/bench_dir_metadata_s3_20260602T042414Z.