ci: speed up builds 4x with parallel tests, coverage, and ccache

Split functional tests into 4 parallel groups and add check-functional-parallel target (~3x faster CI runs).
Add branch-aware LCOV coverage reporting with new --enable-coverage option and lcov summary in CI pipeline.
Enable ccache via -C configure flag for faster recompilations.
Install lcov in all Docker images and use --depth 1 git clones + parallel make -j$(nproc) for quicker container builds.
Update CONTRIBUTING.md with instructions for parallel test groups and adding new ones.

All changes are tightly related CI/performance improvements developed in sequence. No external service uploads (e.g. Codecov skipped due to Gitea incompatibility).

CONTRIBUTING.md

@@ -147,9 +147,28 @@ The functional test suite uses [stabber](https://git.jabber.space/devs/stabber)
 Functional tests require stabber to be installed. Once installed, tests run as part of `make check`:
 
 ```bash
-make check
+make check                      # Run all tests (unit + functional)
+make check-functional-parallel  # Run functional tests in parallel (~3x faster)
+./tests/functionaltests/functionaltests    # Run all functional tests sequentially
+./tests/functionaltests/functionaltests 1  # Run specific group (1-4)
 ```
 
+#### Test groups
+
+Tests are organized into 4 groups for parallel execution:
+
+| Group | Description |
+|-------|-------------|
+| 1 | Connect, Ping, Rooms, Software |
+| 2 | Message, Receipts, Roster, Chat Session |
+| 3 | Presence, Disconnect |
+| 4 | MUC, Carbons |
+
+To add a new group:
+1. Define the test array in `functionaltests.c`
+2. Add entry to `groups[]` array
+3. Update `FUNC_TEST_GROUPS` in `Makefile.am`
+
 #### Writing functional tests
 
 Use content-based stubbing with stabber:
@@ -168,4 +187,5 @@ Guidelines:
 2. Use `stbbr_send(xml)` for presence, message, and other push-style stanzas.
 3. Keep assertions tolerant of ordering when possible; use `prof_output_regex()` for flexible matching.
 4. If timing issues appear, use `prof_timeout()` around critical expectations and reset afterwards.
+5. When adding new tests, place them in the appropriate group based on functionality.