[impl] docs-only fast-pass for required CI

[hevbot]

Closes #35

What changed

• Path-filter the heavy CI workflow so docs-only PRs (docs/**, root *.md) do not run the Rust fmt/clippy/test lane.
• Add a PR-only docs fast-pass twin that classifies changed paths with dorny/paths-filter and reports the same required job name when all changed files are docs.
• Code or mixed diffs still run the existing heavy lane unchanged.

Evidence

• YAML parse: ruby -e 'require "yaml"; Dir[".github/workflows/*.yml"].sort.each { |f| YAML.load_file(f); puts "ok #{f}" }'\n- Workflow lint: actionlint .github/workflows/ci.yml .github/workflows/ci-docs-fastpass.yml\n- Required context proof: both workflow jobs are named exactly fmt / clippy / test (heavy=fmt / clippy / test, fastpass=fmt / clippy / test).\n- This PR changes workflow YAML, so it should exercise the real heavy lane rather than the docs fast-pass.

[hevbot]

Factory verified: paths-filter twin with the exact fmt / clippy / test context name branch protection pins; docs-only diffs green in seconds, code diffs (like this PR itself) run the real lane. Merge-ready pending its own full lane.