testing: document a per-module coverage report to guide new tests

[JohnnyWilson16]

Problem

The 93% coverage gate is enforced globally, so contributors can't easily see which modules are thin and where new tests would be most valuable.

Expected outcome

A documented command (or Makefile target) that prints per-module coverage, plus a short "modules that most want tests" pointer in the contributor roadmap so newcomers know where to aim.

Relevant files

• docs/community/contributor-roadmap.md (add the pointer)
• optionally Makefile (a coverage-report target)

Acceptance criteria

• Running the documented command produces a per-file coverage table (e.g. pytest --cov-report=term-missing-style, scoped to the fast lane).
• The roadmap references it under the Testing tier.
• mkdocs build --strict passes.

Difficulty / skills

good first issue — pytest + coverage basics.