Re-mark the region tracker at the merged order when freeing pages

[cberner]

Summary

Freeing a page could leave the allocator's per-order free-space index (the region tracker) stale, so the database file could grow instead of reusing freed space, and compact() could stop before fully shrinking the file.

Root cause

free_helper() marked the region tracker free only at the freed page's own order. But freeing a page can buddy-merge it into a block of a higher order. Once a region had been marked full at some order K (mark_full, when an allocation of order K failed on it), only a free at order >= K would clear that bit, because mark_free(order) clears orders 0..=order. A free at a lower order that merged up into a free order-K block left the bit set.

With the bit stale, find_free(K) skipped the region even though it held a free block, so allocations grew the file instead of reusing the space, and compact()'s allocate_lowest probe was steered to a higher page and hit its new >= old early break. The buddy allocator itself stayed exact, so no data was corrupted.

Fix

BuddyAllocator::free() now returns the order of the resulting (possibly merged) block, and free_helper() marks the region tracker free at that order. Returning it costs nothing — it is the terminal order of the existing merge recursion — and avoids re-scanning every order to recompute the region's highest free order on each free.

Testing

• Added free_merge_remarks_region_tracker, which fills a region, frees it so the pages merge back up, and asserts the next allocation reuses the region instead of spilling into a new one. Confirmed it fails before the fix and passes after.
• cargo fmt --check, cargo clippy --all-targets --all-features (deny warnings), and cargo test --all-features all pass.

🤖 Generated with Claude Code

https://claude.ai/code/session_016n3Q5CXgzQTFNC8qbPheGp

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Generated by Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 92.30769% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.20%. Comparing base (88881b8) to head (7b3eb1e).

Files with missing lines	Patch %	Lines
src/tree_store/page_store/page_manager.rs	90.90%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1294      +/-   ##
==========================================
- Coverage   90.21%   90.20%   -0.01%     
==========================================
  Files          36       36              
  Lines       16289    16318      +29     
==========================================
+ Hits        14695    14720      +25     
- Misses       1594     1598       +4     

Files with missing lines	Coverage Δ
src/tree_store/page_store/buddy_allocator.rs	96.57% <100.00%> (-0.02%)	⬇️
src/tree_store/page_store/page_manager.rs	95.12% <90.90%> (-0.17%)	⬇️

... and 1 file with indirect coverage changes

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