Preserve markdown download handling

[timkpaine]

Call MarkdownBuilder.finish() from Yardang's explicit yardang-llms builder so sphinx-markdown-builder 0.6.11 can copy referenced downloads.

Without the parent lifecycle hook, direct builder output contains _downloads links but omits the files. Regression coverage builds with yardang-llms and verifies the generated link resolves to a copied file.

[github-actions]

Test Results

84 tests  +1   84 ✅ +1   25s ⏱️ -1s
 1 suites ±0    0 💤 ±0 
 1 files   ±0    0 ❌ ±0 

Results for commit dec2ac9. ± Comparison against base commit 7003778.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.38%. Comparing base (bc1f53d) to head (dec2ac9).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #139      +/-   ##
==========================================
+ Coverage   80.60%   85.38%   +4.78%     
==========================================
  Files          15       15              
  Lines        1897     1909      +12     
  Branches      203      203              
==========================================
+ Hits         1529     1630     +101     
+ Misses        317      216     -101     
- Partials       51       63      +12     

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