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).

Dockerfile.arch

@@ -1,6 +1,7 @@
 FROM archlinux
 
 ENV TERM=xterm
+ENV CC="ccache gcc"
 
 RUN pacman -Syu --noconfirm
 # reflector is optional - if it fails due to network issues, continue with default mirrorlist
@@ -12,9 +13,11 @@ RUN pacman -S --needed --noconfirm \
   autoconf-archive \
   automake \
   base-devel \
+  ccache \
   check \
   cmake \
   cmocka \
+  lcov \
   curl \
   debuginfod \
   doxygen \
@@ -61,12 +64,12 @@ USER root
 RUN pacman -U --noconfirm libstrophe-git/libstrophe-git-*.pkg.tar.zst
 
 WORKDIR /usr/src
-RUN git clone -c http.sslverify=false https://git.jabber.space/devs/stabber
+RUN git clone --depth 1 -c http.sslverify=false https://git.jabber.space/devs/stabber
 
 WORKDIR /usr/src/stabber
 RUN ./bootstrap.sh
 RUN ./configure --prefix=/usr --disable-dependency-tracking
-RUN make
+RUN make -j$(nproc)
 RUN make install
 
 WORKDIR /usr/src/profanity