test(metrics): add edge cases for single timestamp, zero latency, and negative SNR

[realmaxc]

Seven additional tests covering gaps in metrics.py that the existing suite does not reach:

• inter_token_gaps with a single timestamp → empty array
• summarize_latency with one timestamp → zero mean/p95 gaps
• token_accuracy with all correct predictions → 1.0
• token_accuracy without ignore_index → regular accuracy
• first_token_latency when first token arrives at the same instant as the request → 0.0
• percentile with a single observation
• waveform_snr producing a negative dB value when noise dominates

All 38 tests pass locally (pytest tests/test_metrics.py -q).