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.sh

@@ -0,0 +1,21 @@
+#!/usr/bin/env bash
+# How much of lib/ the host tests run: builds them with coverage counters, runs them, and reports.
+# Usage: scripts/coverage.sh [out folder, default .pio/coverage]
+# Out: summary.json (gcovr), coverage.svg (the README's badge), index.html (line by line).
+# What it measures: the lines of lib/ that compile on a PC. Not lib/SD (the card's driver) and not
+# src/ (the Apps, the Services, everything that needs the device): those have no host tests.
+set -euo pipefail
+source "$(dirname "$0")/_docker.sh"
+DOCKER_EXTRA=()
+OUT="${1:-.pio/coverage}"
+
+run_in_container bash -c '
+    set -e
+    git config --global --add safe.directory "$PWD"
+    rm -rf .pio/build/native-coverage "'"$OUT"'"
+    mkdir -p "'"$OUT"'"
+    pio test -e native-coverage | tail -1
+    gcovr -r . --filter "lib/" --object-directory .pio/build/native-coverage \
+        --json-summary "'"$OUT"'/summary.json" --html-details "'"$OUT"'/index.html" --print-summary | tail -4
+    python3 scripts/coverage_badge.py "'"$OUT"'/summary.json" "'"$OUT"'/coverage.svg"
+'