Coverage of lib/ by the host tests, and badges in the README

scripts/coverage.sh builds the host tests with coverage counters and
reports with gcovr: 94.6% of the 3,193 lines of lib/ today (lib/SD and
src/ have no host tests and aren't counted). CI runs it on every push,
puts the figure in the job's summary, and on main publishes a coverage
badge and a latest-release badge to the branch 'badges'. The README shows
them next to Gitea's own badge for the workflow.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EhqxQ49eCju4CzKYNjZzwT

docker/Dockerfile

@@ -5,7 +5,7 @@ RUN apt-get update \
  && apt-get install -y --no-install-recommends git build-essential \
  && rm -rf /var/lib/apt/lists/*
 
-RUN pip install --no-cache-dir platformio
+RUN pip install --no-cache-dir platformio gcovr
 
 # Toolchains and libraries are cached in a named volume mounted here.
 RUN mkdir -p /pio && chmod 777 /pio