feat(runtime): add AAC telemetry calibration

[rylinjames]

Summary

• records adaptive RTC chunking decisions/signals into JSONL request records and OTel span attributes
• adds bounded Prometheus metrics for AAC horizon, risk, guard margin, correction magnitude, uncertainty, and action delta
• adds an offline JSONL/JSONL.gz calibration utility plus script for recommending AAC thresholds from pilot traces

Validation

• PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 PYTHONPATH=src python -m pytest tests/test_adaptive_calibration.py tests/test_observability_prometheus.py tests/test_record.py tests/test_rtc_adapter_day4.py
• PYTHONPATH=src python -m ruff check src/tether/observability/prometheus.py src/tether/observability/init.py src/tether/runtime/server.py src/tether/runtime/adaptive_calibration.py scripts/calibrate_adaptive_chunking.py tests/test_adaptive_calibration.py tests/test_observability_prometheus.py tests/test_record.py tests/test_rtc_adapter_day4.py
• PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 PYTHONPATH=src python -m pytest -p pytest_asyncio.plugin
• PYTHONPATH=src python scripts/calibrate_adaptive_chunking.py --help