Avoid rewriting the parent tree when removing an absent subtree value

[cberner]

Summary

MultimapTable::remove() computed whether the value existed in the subtree but then ran its re-insert block unconditionally. When the removed value was not present, subtree.remove() returned None and left the subtree untouched (delete_key leaves the root untouched when the key isn't found), yet the collection was still re-inserted into the parent tree. That copy-on-wrote the root→leaf path, queued the old pages to be freed, and could even flip the value-set from subtree back to inline storage — all for a logical no-op. The inline arm and Table::remove() already short-circuit correctly.

The fix returns early when the value was not present, matching the inline arm.

Impact

Contents, length, and integrity were already unaffected — this only removes the wasted write amplification for a common operation (removing a value that isn't present from a subtree-backed key).

Testing

• New regression test multimap_remove_nonexistent_value_from_subtree_does_not_churn asserts that a no-op removal on a subtree-backed key allocates and frees zero pages. It fails before the fix (baseline 7 → 10 allocated pages from the transient CoW churn) and passes after.
• Full gate green: cargo fmt --check, cargo clippy --all-targets --all-features, and cargo test --all-features (310 tests, 0 failures).

🤖 Generated with Claude Code

https://claude.ai/code/session_01AHrwK7untocWhi8S4VV9V2

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Generated by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.22%. Comparing base (88881b8) to head (caf2f37).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1295   +/-   ##
=======================================
  Coverage   90.21%   90.22%           
=======================================
  Files          36       36           
  Lines       16289    16320   +31     
=======================================
+ Hits        14695    14724   +29     
- Misses       1594     1596    +2     

Files with missing lines	Coverage Δ
src/multimap_table.rs	93.65% <100.00%> (+0.01%)	⬆️

... and 3 files with indirect coverage changes

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