Propose lint candidates derived from CodeRabbit review findings

[leynos]

Summary

This branch adds RFC 0001, which distils a corpus of 192 CodeRabbit review
findings (83 targeting Rust files) into seven candidate Whitaker lints that
are mechanically detectable, not covered by Clippy or rustc, and not already
scheduled in the roadmap. The RFC recommends three candidates for immediate
scheduling (test_helper_must_return_result, assertion_missing_message,
and no_std_env_operations), two as follow-ups (error_context_discarded
and no_drop_to_silence_unused), and defers two
(fallible_gap_before_guard and parameterizable_duplicate_tests) against
blocking roadmap infrastructure. Clusters already covered by rustc E0761,
clippy::missing_errors_doc, or scheduled roadmap work (§2.2, §7, §8) are
recorded as excluded so the corpus does not need re-analysis.

no_std_env_operations deliberately widens the original test-scoped
environment candidate: direct environment manipulation in tests is a symptom
of production code reading std::env ambiently, so the lint enforces the
injectable-handle pattern (for example &dyn mockable::Env) across
production and test code alike, using no_std_fs_operations as its
implementation template.

Review walkthrough

• Start with
  docs/rfcs/0001-coderabbit-derived-lint-candidates.md
  for the full analysis: the exclusion screen against existing coverage, the
  cluster summary table, per-candidate detection sketches and configuration
  surfaces, and the tiered recommendation.
• Then review
  docs/contents.md
  for the new "Requests for comments" index section linking the RFC.

Validation

• make check-fmt: pass.
• make test: 1453 passed, 3 skipped, 0 failed.
• make typecheck: pass (cargo check --workspace --all-targets --all-features with -D warnings).
• make lint: pass (cargo doc and cargo clippy with -D warnings).
• make markdownlint: 69 files linted, 0 errors.
• make nixie: all Mermaid diagrams validated successfully.

Notes

• The source corpus lives outside the repository
  (~/docs/coderabbit-sample-findings.txt); the RFC records the cluster
  counts and severity profiles so the analysis is reproducible without it.
• An earlier commit on this branch fixing a Mermaid parse error in the suite
  design document was dropped during rebase because the identical fix landed
  on main independently.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch lint-candidates

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