Simplify AllPageNumbersBtreeIter with stack-based traversal

[cberner]

Summary

Refactored AllPageNumbersBtreeIter to use a simpler stack-based traversal approach instead of maintaining complex recursive state machines. This eliminates the RangeIterState enum and related helper functions that were only used by this iterator.

Key Changes

• Removed RangeIterState enum and its three variants (Enter, Leaf, BranchChild) along with all associated methods
• Removed unused helper functions: upper_bound_entry() and leaf_entries()
• Replaced stateful traversal with a pending vector that acts as a stack of page numbers to visit
• Simplified AllPageNumbersBtreeIter::new() to return Self directly instead of Result<Self>
• Updated BtreeMut::all_pages_iter() return type from Result<Option<AllPageNumbersBtreeIter>> to Option<AllPageNumbersBtreeIter>
• Removed fixed_value_size parameter from AllPageNumbersBtreeIter::new() as it's no longer needed
• Updated all call sites in btree.rs and multimap_btree.rs to match the new signature

Implementation Details

The new implementation uses a straightforward depth-first traversal:

• Maintains a stack (pending) of page numbers to visit
• For each page, checks if it's a leaf (terminal) or branch node
• For branch nodes, pushes child page numbers onto the stack in reverse order to maintain left-to-right traversal order
• Returns each page number exactly once without complex state tracking

This approach is more maintainable and easier to understand while providing the same functionality.

https://claude.ai/code/session_01FjAnvpv4K1pPnshfkNX7Pn

[codecov]

Codecov Report

❌ Patch coverage is 68.18182% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.34%. Comparing base (695bfde) to head (f884502).

Files with missing lines	Patch %	Lines
src/tree_store/btree.rs	0.00%	5 Missing ⚠️
src/tree_store/btree_iters.rs	86.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1255      +/-   ##
==========================================
+ Coverage   88.33%   88.34%   +0.01%     
==========================================
  Files          36       36              
  Lines       15022    14848     -174     
==========================================
- Hits        13269    13118     -151     
+ Misses       1753     1730      -23     

Files with missing lines	Coverage Δ
src/multimap_table.rs	84.36% <ø> (+0.09%)	⬆️
src/tree_store/multimap_btree.rs	70.16% <100.00%> (-0.12%)	⬇️
src/tree_store/btree_iters.rs	96.84% <86.66%> (+4.91%)	⬆️
src/tree_store/btree.rs	86.97% <0.00%> (+0.20%)	⬆️

... and 1 file with indirect coverage changes

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