perf(cache): enforce cover byte budget during foreground use

lib/utils/cache_byte_budget.dart

@@ -0,0 +1,41 @@
+import 'dart:io';
+
+/// Trims old cached payloads to a byte budget. Repository metadata, symlinks,
+/// and recently written files (possibly active downloads) are left alone.
+Future<int> trimCacheToByteBudget(
+  Directory directory, {
+  required int maxBytes,
+  required int targetBytes,
+  Duration minimumAge = const Duration(minutes: 1),
+}) async {
+  assert(targetBytes >= 0 && targetBytes <= maxBytes);
+  if (!await directory.exists()) return 0;
+  final files = <(File, FileStat)>[];
+  var total = 0;
+  await for (final entry in directory.list(
+    recursive: true,
+    followLinks: false,
+  )) {
+    if (entry is! File || entry.path.endsWith('.json')) continue;
+    final stat = await entry.stat();
+    if (stat.type != FileSystemEntityType.file) continue;
+    files.add((entry, stat));
+    total += stat.size;
+  }
+  if (total <= maxBytes) return 0;
+  files.sort((a, b) => a.$2.modified.compareTo(b.$2.modified));
+  final cutoff = DateTime.now().subtract(minimumAge);
+  var deleted = 0;
+  for (final (file, stat) in files) {
+    if (total <= targetBytes) break;
+    if (stat.modified.isAfter(cutoff)) continue;
+    try {
+      await file.delete();
+      total -= stat.size;
+      deleted++;
+    } on FileSystemException {
+      // A cache download/eviction may have changed the directory since listing.
+    }
+  }
+  return deleted;
+}