feat: add WriteStats to high-level write operations

[wjones127]

Summary

Add a unified WriteStats struct to track statistics across Insert, Update, Delete, and Merge-Insert operations. This enables users to get information about rows written/updated/deleted, files written, and bytes written.

• InsertBuilder::execute() now returns InsertResult with stats
• UpdateResult now includes WriteStats
• DeleteBuilder::execute() now returns DeleteResult with stats
• MergeStats gains write_stats() accessor for compatibility

Test plan

• Added test_insert_stats to verify insert statistics
• Added test_delete_stats to verify delete statistics
• Added WriteStats assertions to test_update_all
• Added write_stats() accessor test in merge-insert tests
• All 988 lance tests pass

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 93.05019% with 18 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance/src/dataset/write/delete.rs	88.59%	5 Missing and 8 partials ⚠️
rust/lance/src/dataset/write/insert.rs	92.85%	1 Missing and 2 partials ⚠️
rust/lance/src/dataset/fragment.rs	91.66%	0 Missing and 1 partial ⚠️
rust/lance/src/dataset/write.rs	90.90%	0 Missing and 1 partial ⚠️

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