Fix generated Java source links

[nvzm123]

Summary

• make generated Java and Lucene API _Source links point to the documented
  declaration rather than the preceding Javadoc closing line;
• preserve rendered signatures while skipping supported leading annotations,
  whitespace, and Java comments for line selection;
• add focused regression coverage, run it before Fern regeneration, and repair
  the PR docs job's missing changed-files dependency;
• regenerate the affected Java and Lucene API pages.

The generated-page delta is limited to 627 paired _Source replacements across
35 Java and 22 Lucene pages. No generated prose, signatures, navigation, or
ordering changes.

Validation

• python3 -m unittest -v fern.scripts.test_generate_api_reference
• relevant ruff, pydocstyle, whitespace, end-of-file, and
  fern-api-reference pre-commit hooks
• two-run deterministic regeneration
• independent audit of 627 member and 67 class source anchors
• fern/build_docs.sh check (zero errors; all 284 MDX files valid)

Fork CI scheduling and execution had not completed when this draft was opened.
The workflow dependency repair should be confirmed by this PR's first run.