refactor: rename "region" to "shard" in mem_wal implementation

[hamersaw]

Avoid confusion with object store regions (e.g., AWS regions) which are unrelated to the MemWAL concept of a unique writer/reader instance.

Closes #6355

[codecov]

Codecov Report

❌ Patch coverage is 83.50785% with 63 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance-index/src/mem_wal.rs	50.00%	16 Missing and 3 partials ⚠️
rust/lance/src/dataset/mem_wal/manifest.rs	78.43%	9 Missing and 2 partials ⚠️
...ust/lance/src/dataset/mem_wal/scanner/collector.rs	62.06%	11 Missing ⚠️
rust/lance/src/dataset/mem_wal/write.rs	83.33%	10 Missing and 1 partial ⚠️
...t/lance/src/dataset/mem_wal/scanner/data_source.rs	46.15%	7 Missing ⚠️
rust/lance/src/dataset/mem_wal/wal.rs	81.81%	2 Missing ⚠️
rust/lance/src/dataset/mem_wal/scanner/builder.rs	92.85%	1 Missing ⚠️
rust/lance/src/io/commit/conflict_resolver.rs	95.65%	0 Missing and 1 partial ⚠️

📢 Thoughts on this report? Let us know!

[Xuanwo]

Thank you!