feat(runtime): add AAC config canary mode

[rylinjames]

Summary

• adds configurable AAC thresholds to RtcAdapterConfig and tether serve
• adds --adaptive-action-chunking-canary to compute/record AAC decisions while applying the base RTC horizon
• records and exports applied_horizon alongside recommended horizon in stats, spans, metrics, and calibration summaries

Validation

• PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 PYTHONPATH=src python -m pytest -p pytest_asyncio.plugin tests/test_rtc_adapter_day1.py tests/test_rtc_adapter_integration.py tests/test_observability_prometheus.py tests/test_adaptive_calibration.py tests/test_rtc_adapter_day4.py
• PYTHONPATH=src python -m ruff check src/tether/runtime/rtc_adapter.py src/tether/observability/prometheus.py src/tether/runtime/server.py src/tether/runtime/adaptive_calibration.py tests/test_rtc_adapter_day1.py tests/test_rtc_adapter_integration.py tests/test_observability_prometheus.py tests/test_adaptive_calibration.py tests/test_rtc_adapter_day4.py
• PYTHONPATH=src python -m py_compile src/tether/cli.py
• Typer parser acceptance for --adaptive-action-chunking-canary
• PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 PYTHONPATH=src python -m pytest -p pytest_asyncio.plugin