Teach deps noise suppression about Markdown heading explosions

[Widthdom]

Summary

deps --suppress-noise removes zero evidence from the observed Markdown heading-name edges, including edges with thousands of unrelated references.

Audit IDs: G-13. Reproduced with locally built cdidx v1.40.3 at 404db09489c305f17d94f37026e00564d3ac527a.

Existing issue relationship

Residual/follow-up to closed #4113. Complementary to open #4846, which fixes semantic Markdown link/heading resolution for new indexes; this issue keeps suppression useful for existing or otherwise noisy indexes.

Expected behavior and PR scope

Noise suppression should identify heading-derived fan-out by evidence/provenance and suppress it without hiding explicit Markdown links. Keep this to the deps suppression classifier/output.

Implementation guide for Codex 5.6 sol xhigh

• Expose reference origin/kind/count distribution needed to distinguish heading co-occurrence from explicit links.
• Suppress or down-rank synthetic same-heading fan-out and report before/after counts/reasons.
• Test old/noisy-index fixtures as well as fresh semantic-link fixtures from Resolve Markdown links to heading declarations without heading-name dependency explosions #4846.
• Avoid filename-only or common-word blacklists.
• Add focused regression/contract tests, run the full net8.0 and net9.0 suite, and add the required bilingual changelog.d/unreleased/ fragment.

Acceptance criteria

• The reproduced huge heading edges are removed or clearly isolated under suppression.
• Real explicit Markdown links remain dependencies.
• JSON reports what was suppressed and why.

[Widthdom]

Work started on branch fix-issue4868.