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

platformio.ini

@@ -46,3 +46,14 @@ build_flags =
 platform = native
 lib_ldf_mode = deep+
 build_flags = -std=gnu++17
+
+; The same tests, built to count which lines of lib/ they run (scripts/coverage.sh).
+[env:native-coverage]
+extends = env:native
+build_flags =
+    ${env:native.build_flags}
+    --coverage
+    -O0
+extra_scripts =
+    ${env.extra_scripts}
+    pre:scripts/coverage_link.py