drivers/relays: hardened + 100% MC/DC (Tier-1, first driver)

Bring relays.c into warden-sdk/drivers/ behind a relay_io seam (ADR-0005): the
sysfs plumbing is an injectable backend, the gpio root is $WARDEN_GPIO_ROOT-
overridable. Host harness (drivers/relays/test/) reaches 100% MC/DC (40/40
conditions, 100% lines) by combining fake-io unit tests (logic branches incl.
export->node-appears) with real-sysfs integration tests over a scratch tree
(backend fopen/stat branches). CI `mcdc` job enforces 100% across drivers/*/test.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017wB8KB3MMQztRDXCMCkPrf

drivers/relays/test/enforce-mcdc.sh

@@ -0,0 +1,48 @@
+#!/usr/bin/env bash
+# Gate: fail unless every unit check passed AND relays.c reached 100% MC/DC
+# (condition) coverage with no uncovered executable lines.
+#   $1 = gcov stdout log   $2 = relays.c.gcov   $3 = test exit-code file
+set -uo pipefail
+LOG="$1"; GCOV="$2"; RCFILE="$3"
+rc=0
+
+testrc="$(cat "$RCFILE" 2>/dev/null || echo 1)"
+if [ "$testrc" != "0" ]; then
+	echo "RESULT: unit checks FAILED (test exit $testrc)"; rc=1
+else
+	echo "RESULT: all unit checks passed"
+fi
+
+if [ ! -f "$GCOV" ]; then
+	echo "RESULT: no coverage file ($GCOV) produced"; exit 1
+fi
+
+notcov="$(grep -nE "condition[s]? .*not covered" "$GCOV" || true)"
+uncov_lines="$(grep -nE "^ +#####:" "$GCOV" || true)"
+
+# Summary lines for relays.c from gcov stdout. Match the exact file so
+# 'test_relays.c' (which also contains "relays.c") is NOT picked up.
+FMATCH="File '([^']*/)?relays[.]c'"
+cond_line="$(awk -v patt="$FMATCH" '$0 ~ patt {f=1} f&&/Condition outcomes covered:/{print; f=0}' "$LOG")"
+line_line="$(awk -v patt="$FMATCH" '$0 ~ patt {f=1} f&&/Lines executed:/{print; f=0}' "$LOG")"
+echo "  ${line_line:-Lines executed: (n/a)}"
+echo "  ${cond_line:-Condition outcomes covered: (n/a)}"
+
+if [ -n "$notcov" ]; then
+	echo "RESULT: MC/DC gaps (conditions not covered):"
+	echo "$notcov" | sed 's/^/    /'
+	rc=1
+fi
+if [ -n "$uncov_lines" ]; then
+	echo "RESULT: uncovered executable lines in relays.c:"
+	echo "$uncov_lines" | sed 's/^/    /'
+	rc=1
+fi
+
+if ! echo "$cond_line" | grep -q "100.00%"; then
+	echo "RESULT: condition coverage is below 100%"
+	rc=1
+fi
+
+[ "$rc" = "0" ] && echo "RESULT: 100% MC/DC + all checks green ✓"
+exit "$rc"