Fix edit algorithms deleting the wrong child via remove_child(<pointer>)

[TroyHernandez]

Composition only declares remove_child(int index, ErrorStatus*), but overwrite() and insert() in editAlgorithm.cpp call it with a Composable* / Retainer:

composition->remove_child(transition);     // overwrite() and insert()
composition->remove_child(items.back());   // overwrite()

The pointer argument decays to bool (always true) → int(1), so these calls remove the child at index 1 (or, via adjusted_vector_index, the last child when size() == 1), not the item that was passed.

Impact

• overwrite() over a multi-item span deletes the wrong clips: the partially-overwritten clip is removed while a fully-overwritten clip survives in place.
• Transition removal in overwrite()/insert() removes index 1 instead of the intended transition.

Repro

Track A|M|B (each 5/8/5 frames), overwrite(X, track, [8, 18)) — a span that partially covers M and fully covers B to the track end:

before (buggy):  A[0+5] B[20+5] X[2+10]   # M removed, B kept — wrong
after  (fixed):  A[0+5] M[10+3] X[2+10]   # M trimmed, B removed — correct

Fix

Pass the already-computed (and bounds-checked) index for transitions, and index_of_child(items.back()) in the removal loop.

Tests

• Adds test_edit_overwrite_partial_first_full_last, which fails before this change and passes after.
• The existing test_editAlgorithm suite continues to pass.

I verified the fix and the regression test by building the library locally (the test fails on the unfixed tree, passes on the fixed tree).

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: TroyHernandez / name: Troy Hernandez (8680cd2)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.50%. Comparing base (6944f10) to head (8680cd2).

Files with missing lines	Patch %	Lines
src/opentimelineio/algo/editAlgorithm.cpp	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2025      +/-   ##
==========================================
- Coverage   84.51%   84.50%   -0.01%     
==========================================
  Files         181      181              
  Lines       13239    13240       +1     
  Branches     1202     1202              
==========================================
  Hits        11189    11189              
- Misses       1867     1868       +1     
  Partials      183      183              

Flag	Coverage Δ
py-unittests	84.50% <0.00%> (-0.01%)	⬇️

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Files with missing lines	Coverage Δ
src/opentimelineio/algo/editAlgorithm.cpp	0.00% <0.00%> (ø)

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