chore(coverage): relax thresholds to cut low-value churn

Lower the diff-coverage gate from 90% to 80% and the critical-module
target from 90% to 85%. The 90% diff gate forced back-fill tests on
nearly every changed line; 80% keeps new code honest without the churn.
Global coverage stays informational per ADR 0004 (no new gate added).

Updates scripts/diff_coverage.py, scripts/coverage.sh,
scripts/critical-modules.txt, .gitea/workflows/test.yml, and records the
change as a dated revision in ADR 0004.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

scripts/coverage.sh

@@ -13,7 +13,7 @@
 #     are re-executed; no KVM or Docker dependency.
 #
 # Pass "critical" as the last argument in either mode to also report just the
-# critical modules (ADR 0004 target: 90%).
+# critical modules (ADR 0004 target: 85%).
 set -euo pipefail
 
 cd "$(dirname "$0")/.."
@@ -34,8 +34,8 @@ if [ "${1:-}" = "aggregate" ]; then
     "$PY" -m coverage report -m
 
     if [ "${2:-}" = "critical" ]; then
-        echo "== critical modules (ADR 0004 minimum: 90%) ==" >&2
-        "$PY" -m coverage report --include="$CRITICAL" --fail-under=90
+        echo "== critical modules (ADR 0004 minimum: 85%) ==" >&2
+        "$PY" -m coverage report --include="$CRITICAL" --fail-under=85
     fi
     exit 0
 fi
@@ -55,6 +55,6 @@ echo "== combined report ==" >&2
 "$PY" -m coverage report -m
 
 if [ "${1:-}" = "critical" ]; then
-    echo "== critical modules (ADR 0004 minimum: 90%) ==" >&2
-    "$PY" -m coverage report --include="$CRITICAL" --fail-under=90
+    echo "== critical modules (ADR 0004 minimum: 85%) ==" >&2
+    "$PY" -m coverage report --include="$CRITICAL" --fail-under=85
 fi

scripts/critical-modules.txt

@@ -1,4 +1,4 @@
-# Critical security/logic core held to the >=90% coverage bar by
+# Critical security/logic core held to the >=85% coverage bar by
 # docs/decisions/0004-coverage-policy.md.
 #
 # SINGLE SOURCE OF TRUTH: scripts/coverage.sh (the `critical` report) and

scripts/diff_coverage.py

@@ -13,8 +13,8 @@ policy.
 
 Usage:
     scripts/coverage.sh                 # produce .coverage first
-    python3 scripts/diff_coverage.py    # gate against origin/main, min 90%
-    python3 scripts/diff_coverage.py --base main --min 85
+    python3 scripts/diff_coverage.py    # gate against origin/main, min 80%
+    python3 scripts/diff_coverage.py --base main --min 75
 """
 
 from __future__ import annotations
@@ -74,7 +74,7 @@ def main() -> int:
     ap = argparse.ArgumentParser()
     ap.add_argument("--base", default="origin/main",
                     help="git ref to diff against (default: origin/main)")
-    ap.add_argument("--min", type=float, default=90.0,
+    ap.add_argument("--min", type=float, default=80.0,
                     help="minimum %% of changed executable lines covered")
     args = ap.parse_args()