Add .loop-build harness and IntelliJ project configuration files

.loop-build/scripts/apply_patch.sh

@@ -0,0 +1,38 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+ROOT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")/../.." && pwd)"
+
+if [[ ! -d "$ROOT_DIR/.git" ]]; then
+  echo "[apply_patch][ERROR] no git repository available"
+  exit 2
+fi
+
+patch_tmp=$(mktemp)
+cleanup() { rm -f "$patch_tmp"; }
+trap cleanup EXIT
+
+if [[ $# -ge 1 ]]; then
+  cp "$1" "$patch_tmp"
+else
+  cat > "$patch_tmp"
+fi
+
+if [[ ! -s "$patch_tmp" ]]; then
+  echo "[apply_patch][ERROR] patch is empty"
+  exit 2
+fi
+
+if ! git -C "$ROOT_DIR" apply --check "$patch_tmp" >/dev/null 2>&1; then
+  echo "[apply_patch][ERROR] patch does not apply cleanly"
+  exit 2
+fi
+
+file_count=$(git -C "$ROOT_DIR" apply --numstat "$patch_tmp" | awk '$3 != "" {print $3}' | sort -u | wc -l | tr -d ' ')
+if (( file_count > 2 )); then
+  echo "[apply_patch][BLOCK] patch touches $file_count files; max is 2 per step."
+  exit 2
+fi
+
+git -C "$ROOT_DIR" apply "$patch_tmp"
+echo "[apply_patch][OK] patch applied"