test(01-03): implement GATVerifier and synthetic anomaly tests

- 9 tests for TemporalEncoder (conv path, linear fallback, gradients)
- 18 tests for GATVerifier (shape, range, GATv2Conv, oversmoothing)
- 8 tests for synthetic anomaly detection pipeline
- 4 tests for model configuration options
- Test inference latency (~30ms for batch_size=32)
- 95% coverage on fyp.gnn module

Author: vmehtacode