provider-test-coverage: show last-tested time + who ran it

Each pair in the readiness report now shows a freshness line:
'last tested 9 minutes ago (2026-05-30) by developer (dev build)'.

- humanize_time_ago(): plain-English relative time (just now / minutes /
  hours / days / months / years) plus the absolute date; clamps future
  timestamps to 'just now'.
- coverage_actor_label(): durable, evidence-based actor classification from
  the per-run build flag/version -- clean release build => 'user (release
  build)', dirty/dev build => 'developer (dev build)'.
- Pair builder now tracks the latest entry's jcode_version + dirty flag;
  added latest_jcode_version/latest_jcode_dirty to the pair struct (serde
  default for back-compat with existing ledgers).
- Single-pair query view (format_provider_test_coverage_report) gets the same
  humanized 'Last tested ... by ...' line.
- Colorizer dims the metadata line (90) and keeps 'READY -- ...' green.
- Tests: humanize_time_ago_buckets_and_labels_actor covers all buckets +
  actor cases; existing live_tests::tests all pass.