Add regression test harness and baseline metrics

docs/regression-baseline.txt

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+Generated file count: 932
+Total build errors: 733578
+Cannot find errors (dependency, ignored): 268626
+Fixable build errors: 464952
+Lint errors: 0
+
+Note: The current baseline shows a regression. The dominant error
+(57,784 occurrences) is "multiple incompatible access-level modifiers
+specified" coming from 31 files that contain `public open class`.
+In Swift 6.3, `public` and `open` cannot be combined — `open` is
+itself an access level modifier. The regression was introduced by
+commit ffe6e10 "Force open class for classes with same-namespace
+subclasses" which added `open class` but kept the unconditional
+`public ` prefix in CodeGen.swift.

scripts/regression-test.sh

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+#!/usr/bin/env bash
+# Regression test for swift-gtk-gen output.
+# Run this after any change to CodeGen.swift to catch regressions.
+
+set -euo pipefail
+
+cd "$(dirname "$0")/.."
+
+TEST_DIR="/tmp/regression-test-$$"
+echo "=== swift-gtk-gen regression test ==="
+echo "Test output directory: $TEST_DIR"
+
+# Generate
+rm -rf "$TEST_DIR"
+mkdir -p "$TEST_DIR"
+.build/x86_64-unknown-linux-gnu/debug/swift-gtk-gen \
+    --gir-file /usr/share/gir-1.0/Gtk-4.0.gir \
+    --output "$TEST_DIR" \
+    --generate-all 2>/dev/null
+
+# Run unit tests
+echo ""
+echo "=== Unit tests ==="
+swift test 2>&1 | tail -2
+
+# Count errors
+echo ""
+echo "=== Generated output metrics ==="
+file_count=$(find "$TEST_DIR" -name "*.swift" | wc -l)
+echo "Generated Swift files: $file_count"
+
+cd "$TEST_DIR"
+if swift build 2>&1 > /tmp/build-output.log; then
+    echo "Generated code BUILD: PASS"
+else
+    cannot_find=$(grep "error:" /tmp/build-output.log | grep -c "cannot find" || echo 0)
+    total=$(grep -c "error:" /tmp/build-output.log || echo 0)
+    fixable=$((total - cannot_find))
+    echo "Generated code BUILD: FAIL ($total total errors, $cannot_find dependency-ignored, $fixable fixable)"
+    if [ "$fixable" -gt 0 ]; then
+        echo "First 20 fixable errors:"
+        grep "error:" /tmp/build-output.log | grep -v "cannot find" | head -20
+        exit 1
+    fi
+fi
+
+# Lint
+echo ""
+echo "=== Lint ==="
+cd -
+lint_errors=$(swift format lint --configuration .swift-format --recursive "$TEST_DIR" 2>&1 | grep -c "error:" || echo 0)
+echo "Lint errors: $lint_errors"
+if [ "$lint_errors" -gt 0 ]; then
+    echo "First 10 lint errors:"
+    swift format lint --configuration .swift-format --recursive "$TEST_DIR" 2>&1 | grep "error:" | head -10
+fi
+
+# Cleanup
+rm -rf "$TEST_DIR"
+echo ""
+echo "=== Done ==="