[FEATURE] Coverage Phase 3: Unit 80% / Integration 55%

[sergio-sisternes-epam]

Is your feature request related to a problem? Please describe.
After Phase 2, only unit coverage is gated. Integration coverage can
still regress freely, and the unit gate has 5 points of slack.

Describe the solution you'd like
Tighten unit gate to match baseline (80%) and introduce the first
integration gate at 55% -- 6 points above the 49% baseline.

Changes

1. Raise unit fail_under to 80 in pyproject.toml
2. Add integration coverage gate at 55% in ci-integration.yml
   fan-in job (exit 1 if merged coverage < 55%)
3. Apply ratchet rule to both tiers

Describe alternatives you've considered

• Skipping straight to 70% integration -- rejected because the 49%
  baseline requires meaningful gap-fill first; 55% is achievable
  with modest targeted effort.

Additional context
Parent: #1398 -- Phase 3 of the progressive coverage strangler-fig ratchet.

The unit gate now matches the current baseline -- any regression in
unit coverage will block the PR. The 55% integration target is 6 points
above the 49% baseline, requiring modest gap-fill in integration tests
for the least-covered modules.

Acceptance criteria

• CI unit job fails if coverage < 80%
• Integration fan-in job fails if merged coverage < 55%
• Coverage summaries show gate status in $GITHUB_STEP_SUMMARY
• No test-only changes that game coverage (e.g. excluding modules)