test: include integration test coverage in coverage gate

- Add lcov to nix flake (required for flutter --merge-coverage)
- stalwart-dev/test.sh: collect and merge coverage when unit baseline exists
- run_unit_tests.sh: remove inline coverage check (now in dedicated task)
- Taskfile: add coverage task; check runs test → integration → coverage
  sequentially so the gate sees combined unit + integration data
- check-fast (pre-commit) omits coverage gate since integration tests
  don't run there; full gate runs only in task check
- Drop two untestable fake-only tests (UID-validity reset, malformed envelope)
- Coverage threshold restored to 80% (84% with merged data)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Taskfile.yml

@@ -131,10 +131,19 @@ tasks:
     cmds:
       - fvm flutter run -d linux --no-pub
 
+  coverage:
+    desc: Coverage gate — run after test (and optionally integration) have written lcov.info
+    deps: [_nix-check]
+    cmds:
+      - fvm dart run scripts/check_coverage.dart
+
   check-fast:
     desc: Pre-commit checks — analyze + unit tests + widget tests (no build, no integration)
     deps: [analyze, test, test-widget]
 
   check:
-    desc: Full check suite — analyze + unit tests + widget tests + build-linux + integration in parallel
-    deps: [analyze, test, test-widget, build-linux, integration]
+    desc: Full check suite — unit tests first, then integration (merges coverage), then gate
+    deps: [analyze, test-widget, build-linux, test]
+    cmds:
+      - task: integration
+      - task: coverage