feat(coverage): redesign provider-test-coverage as an 11-stage pipeline view

Rewrites the provider-test-coverage report around the same 11 strict
checkpoints provider-doctor drives, presented as a left-to-right pipeline.

- Headline READY x/y (%) up top; one sentence that 'not READY' means
  work-in-progress, not broken.
- Numbered pipeline legend (live catalog .. real tool smoke).
- Per-provider rollup shows READY count plus how far the best non-ready
  pair climbed (stage number + name) instead of opaque smoke columns.
- Per-pair one-line bar (+ pass, x fail, ! blocked, ~ skip, . not run)
  sorted furthest-first, so the shape of each pair's progress is obvious
  at a glance.
- For every non-ready pair, names the first blocker in plain English and
  prints the exact `jcode provider-doctor <provider> --model <m> --tier
  <t>` command to advance it (or notes when provider-doctor can't drive
  that provider yet).
- Tightens issue-tracked targets and known-empty providers to compact
  lines; adds a Next steps footer pointing at provider-doctor + docs.

Drops the old prose-heavy legend and the raw checkpoint-id gap dumps.
Updates colorizer and the two affected unit tests; all 7 live_tests
unit tests pass.