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

scripts/coverage_link.py

@@ -0,0 +1,4 @@
+# The coverage build must also link with --coverage (build_flags only reaches the compiler).
+Import("env")  # noqa: F821  (provided by PlatformIO)
+
+env.Append(LINKFLAGS=["--coverage"])  # noqa: F821