docs: add visual decision tree, density-bar heatmap, and CI link-check

[Lightheartdevs]

Summary

Phase 3 of the docs reorganization. Stacked on top of #13 — change the base after that one merges.

Visual aids in COMPARISON.md plus a CI guard against future link drift.

• `COMPARISON.md`: new Mermaid decision tree right after TL;DR. Linear flowchart for the main path (harm-sensitive? → research? → bounded? → default) plus a dotted special-case branch (long-horizon or high-stakes → none of these single-shot). Renders natively on GitHub.
• `COMPARISON.md`: replaced the Headline ship rates table with a unicode density-bar version. Each bar is 10 segments regardless of N, so N=3 cells stay visually comparable to N=10 cells; cell winners are obvious at-a-glance.
• New `.github/workflows/check-links.yml` — GitHub Actions workflow that runs the same broken-relative-link scan used during Phase 1/2 verification. Triggers on PRs touching .md files and on pushes to main. Stdlib-only Python, no dependency install. Will fail this very PR's CI if any internal links break.

Skipped from the original plan (with reasons):

• PNG charts via matplotlib — not in environment, and Mermaid + unicode bars are text-editable which is better for a maintainer updating numbers later.
• `microbench-2026-04-28` ↔ phase-b consolidation — Phase 1 cross-pointers already address this.
• PASS-rate grader sweep on no-think tarballs — that's a benchmarking task, not a docs task. Should be its own session; would let COMPARISON.md drop the "presumed equivalent" caveat on the harder cells.

Test plan

• Render check on the Mermaid diagram — open COMPARISON.md on GitHub and confirm the decision tree renders. If GitHub's mermaid version doesn't like `
  ` tags inside node text or the `>` HTML entity, may need to reformat.
• Visually scan the unicode density-bar heatmap — winners should pop out
• Watch this PR's Actions tab — the new `check-links` workflow should run and pass with `checked=831 missing=0`
• Intentionally break a link in a follow-up commit to this PR; confirm the workflow fails. Revert before merge.

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