CI: a push runs the tests, a pull request also builds the firmware

Rebuilding both firmwares on every push was more than anyone looked at.
A push now runs the host tests with their coverage (under two minutes);
a pull request adds the release firmware and the Debug Build, and is how
changes reach main; a tag still does everything before it releases.
Pull requests from forks don't run. scripts/ci.sh takes 'tests' or
'builds' for one half; coverage.sh now fails when a test fails.

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

scripts/ci.sh

@@ -1,7 +1,14 @@
 #!/usr/bin/env bash
 # Local CI: run host unit tests, then build the firmware.
+# Usage: scripts/ci.sh [tests|builds]   one half only; both by default
 set -euo pipefail
 source "$(dirname "$0")/_docker.sh"
 DOCKER_EXTRA=()
 
-run_in_container bash -c 'git config --global --add safe.directory "$PWD" && pio test -e native && pio run -e cardputer-adv -e cardputer-adv-debug'
+case "${1:-all}" in
+    tests) STEPS='pio test -e native' ;;
+    builds) STEPS='pio run -e cardputer-adv -e cardputer-adv-debug' ;;
+    all) STEPS='pio test -e native && pio run -e cardputer-adv -e cardputer-adv-debug' ;;
+    *) echo "Usage: scripts/ci.sh [tests|builds]" >&2; exit 1 ;;
+esac
+run_in_container bash -c 'git config --global --add safe.directory "$PWD" && '"$STEPS"