fix: preserve authored $ref+sibling syntax on render (issue #575)

[daveshanley]

When the sibling-ref transformer rewrites a $ref with sibling fields into a synthetic allOf, the model still rendered that allOf back out, changing the authored syntax. Add renderTransformedRefWithSiblings to collapse the synthetic two-element allOf back into the original $ref+sibling shape on MarshalYAML, reusing the low-level TransformedRef node for key ordering and merging in mutated sibling values.

Only the transformer-created allOf is collapsed; authored allOf blocks (or outer schema mutations) keep their explicit allOf so no changes are hidden. Add unit coverage for the new render path and helpers, an issue-575 regression spec, and update document render tests to assert byte-exact round-tripping.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.69%. Comparing base (5fd8f4f) to head (cc47bc6).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #576      +/-   ##
==========================================
- Coverage   99.72%   99.69%   -0.03%     
==========================================
  Files         280      280              
  Lines       33575    33666      +91     
==========================================
+ Hits        33483    33564      +81     
- Misses         55       64       +9     
- Partials       37       38       +1     

Flag	Coverage Δ
unittests	99.69% <100.00%> (-0.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.