fix(what-changed): compare object composition refactors safely

[daveshanley]

Object schemas moved into an equivalent oneOf or a wider anyOf currently produce false breaking property removals. Compare the preserved object branch through the existing schema comparator, while keeping added alternatives visible as non-breaking changes.

Rebuilds #611 and retains its regression scenarios. The shortcut is limited to pure wrappers, explicit object/null types, and provably disjoint object-versus-null oneOf branches. Wrapper assertions, overlapping branches, dropped types, changed root enum/const values, and uncertain cases retain structural comparison. Null alternatives account for enum and const constraints.

Validation: Linux and Windows full test suites, the Linux race suite, and CodeQL passed on ccf29e001db1b5e7f0d291aa1a11b0835710d1e0. Codecov reports 100% patch and project coverage. Public CompareDocuments regressions cover the original blocking cases, real widening, null constraints, and structured enum narrowing. Independent correctness, de-slop, and final repair reviews are complete with no open findings.

Risk: medium. This changes diff classification, with conservative fallback for unsupported shapes. No public API or dependency changes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (f7414c2) to head (ccf29e0).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #650   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          300       301    +1     
  Lines        38284     38373   +89     
=========================================
+ Hits         38284     38373   +89     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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