support merging multiple MassMap shards into one file

[SF-Zhou]

No description provided.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (bedd376) to head (7061820).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

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##              main        #8    +/-   ##
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  Coverage   100.00%   100.00%            
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  Files            6         6            
  Lines          676       898   +222     
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+ Hits           676       898   +222     

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[Copilot]

Pull Request Overview

This pull request adds a new MassMapMerger feature to efficiently merge multiple massmap files without full deserialization. The merger enables combining disjoint bucket ranges from multiple massmaps into a single consolidated file.

Key changes:

• Introduced MassMapMerger to merge multiple massmap files
• Replaced empty_buckets with occupied_bucket_count and added occupied_bucket_range to track non-empty bucket distribution
• Made bucket_metas and reader fields accessible within the crate to support merging

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/meta.rs	Updated MassMapMeta to track occupied buckets with count and range instead of empty bucket count
src/massmap.rs	Changed visibility of bucket_metas and reader from private to pub(crate) to enable merge operations
src/lib.rs	Exported MassMapMerger as a public API
src/builder.rs	Added MassMapMerger implementation with merge logic, updated builder to track occupied bucket ranges, and added comprehensive test coverage
examples/massmap.rs	Added CLI merge command with argument handling
README.md	Updated documentation examples to reflect the new occupied_bucket_count field

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