feat: implement DoubleEndedIterator for tree iterators

[NikhilSharmaWe]

Closes #102

Implements DoubleEndedIterator for Iter and InterfaceIter, so list.iter().rev() and next_back() work.

The backward walk mirrors the existing forward iterator: separate stack, same bit-based descent, same PackedLeaf chunk boundary handling in reverse. Pulled the forward logic into a few helpers (descend_to_target, advance_front, retreat_back) rather than duplicating the match arms.

InterfaceIter needed a bit of extra care. When updates are pending, logical length can be ahead of what's actually in the tree (e.g. push without apply). Skipping tree_iter.next_back() for indices past tree_backing_len fixes reverse iteration in that case. Forward does the same skip now too.

Test plan

• unit tests for .rev(), partial lists, iter_from().rev(), mixed next/next_back, pending push, updates
• proptest IterRev / IterFromRev

[codecov-commenter]

Codecov Report

❌ Patch coverage is 88.28829% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.32%. Comparing base (8963bc5) to head (0f501f2).

Files with missing lines	Patch %	Lines
src/iter.rs	86.17%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #103      +/-   ##
==========================================
+ Coverage   70.15%   72.32%   +2.16%     
==========================================
  Files          22       22              
  Lines        1280     1355      +75     
==========================================
+ Hits          898      980      +82     
+ Misses        382      375       -7     

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[NikhilSharmaWe]

@michaelsproul Could you please take a look?