Build stateless LabelKeyResolver with fallback logic

The default English labels map should be populated by copying the constant values from LabelKeyRegistry and providing human-readable English defaults. Do not use LabelKeyRegistry constants as the default value — the constant is a key, not a label. Maintain a separate `_defaultLabels` map. For the formatted-key fallback, implement a simple `_formatKey(String key)` helper that splits on `.` and `_`, capitalizes each word, and joins with spaces — this ensures the UI is always readable even during development before all labels are configured.

For parameterized substitution, use `String.replaceAllMapped(RegExp(r'\{\{(\w+)\}\}'), (m) => params?[m.group(1)] ?? m.group(0)!)` — compile the RegExp as a static field to avoid re-compilation. Keep this class free of any Flutter imports — it is pure Dart and must be testable without a Flutter environment.

Unit tests in `label_key_resolver_test.dart` using `flutter_test`. Group by behavior: (1) fallback chain — test each of the 3 fallback levels independently, (2) parameterized substitution — test single param, multiple params, missing param leaves placeholder, null params map treated as empty, (3) edge cases — empty string in labels map falls through, null-safe behavior with empty maps, key not in LabelKeyRegistry returns formatted fallback. Use `const` test data to keep tests fast and deterministic. Assert exact output strings, not just non-null.

Target: all 3 branches of the fallback chain explicitly tested.