ci: improve CI stability with parallel builds and Valgrind

Major changes:
Run 4 build configurations in parallel with Valgrind on Linux
Add test failure detection verification (meta-test)
Port allocation per build to prevent conflicts in parallel runs
Add --coverage-only flag for dedicated coverage builds
Code quality:

Add TEST_GROUPS constant, CMOCKA patterns, helper functions
Organize ci-build.sh into sections

CONTRIBUTING.md

@@ -90,6 +90,24 @@ set -e
 ```
 
 This will run the same tests that the CI runs and refuse the push if it fails.
+The CI script runs 4 parallel builds with different configurations:
+- **Full** — all features enabled (+ coverage in `--coverage-only` mode)
+- **Minimal** — all optional features disabled
+- **NoEncrypt** — no encryption (OTR, PGP, OMEMO disabled)
+- **Default** — default ./configure options
+
+Each build runs Valgrind and functional tests on Linux.
+Use `./ci-build.sh --coverage-only` to run only the Full build with coverage collection.
+
+Output shows test results per build:
+```
+✓ Full PASSED
+  Unit tests:       437 passed, 0 failed
+  Functional tests: 69 passed, 0 failed
+  Coverage:         Lines: 27.5% | Functions: 36.2% | Branches: 18.1%
+  Duration:         5m39s
+```
+
 Note that it will run on the actual content of the repository directory and not
 what may have been staged/committed.