chore: add local pre-CI patch report generation for backend and frontend coverage

- Implemented a new script `local-patch-report.sh` to generate a local patch report.
- The report computes patch coverage based on changes from the current branch against `origin/main`.
- Integrated backend and frontend coverage inputs, producing both Markdown and JSON output artifacts.
- Updated existing frontend coverage script to validate the presence of LCOV coverage file.
- Added tests for coverage computation and parsing of unified diffs for changed lines.
- Enhanced error handling and validation for coverage inputs and baseline references.

scripts/frontend-test-coverage.sh

@@ -28,12 +28,18 @@ mkdir -p coverage/.tmp
 npm run test:coverage -- --run
 
 SUMMARY_FILE="coverage/coverage-summary.json"
+LCOV_FILE="coverage/lcov.info"
 
 if [ ! -f "$SUMMARY_FILE" ]; then
     echo "Error: Coverage summary file not found at $SUMMARY_FILE"
     exit 1
 fi
 
+if [ ! -f "$LCOV_FILE" ]; then
+    echo "Error: LCOV coverage file not found at $LCOV_FILE"
+    exit 1
+fi
+
 # Extract coverage metrics and validate
 LINES_PERCENT=$(python3 - <<'PY'
 import json

scripts/local-patch-report.sh

@@ -0,0 +1,52 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+ROOT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+BASELINE="${CHARON_PATCH_BASELINE:-origin/main...HEAD}"
+BACKEND_COVERAGE_FILE="$ROOT_DIR/backend/coverage.txt"
+FRONTEND_COVERAGE_FILE="$ROOT_DIR/frontend/coverage/lcov.info"
+JSON_OUT="$ROOT_DIR/test-results/local-patch-report.json"
+MD_OUT="$ROOT_DIR/test-results/local-patch-report.md"
+
+if ! command -v git >/dev/null 2>&1; then
+    echo "Error: git is required to generate local patch report." >&2
+    exit 1
+fi
+
+if ! command -v go >/dev/null 2>&1; then
+    echo "Error: go is required to generate local patch report." >&2
+    exit 1
+fi
+
+if [[ ! -f "$BACKEND_COVERAGE_FILE" ]]; then
+    echo "Error: backend coverage input missing at $BACKEND_COVERAGE_FILE" >&2
+    exit 1
+fi
+
+if [[ ! -f "$FRONTEND_COVERAGE_FILE" ]]; then
+    echo "Error: frontend coverage input missing at $FRONTEND_COVERAGE_FILE" >&2
+    exit 1
+fi
+
+BASE_REF="$BASELINE"
+if [[ "$BASELINE" == *"..."* ]]; then
+    BASE_REF="${BASELINE%%...*}"
+fi
+
+if [[ -n "$BASE_REF" ]] && ! git -C "$ROOT_DIR" rev-parse --verify --quiet "${BASE_REF}^{commit}" >/dev/null; then
+    echo "Error: baseline base ref '$BASE_REF' is not available locally. Set CHARON_PATCH_BASELINE to a valid range and retry." >&2
+    exit 1
+fi
+
+mkdir -p "$ROOT_DIR/test-results"
+
+(
+    cd "$ROOT_DIR/backend"
+    go run ./cmd/localpatchreport \
+        --repo-root "$ROOT_DIR" \
+        --baseline "$BASELINE" \
+        --backend-coverage "$BACKEND_COVERAGE_FILE" \
+        --frontend-coverage "$FRONTEND_COVERAGE_FILE" \
+        --json-out "$JSON_OUT" \
+        --md-out "$MD_OUT"
+)