Prevent voice command prompt echo cancel

[Anthonyiswhy]

Summary

BlindNav Safety Checklist

• No code path sends SIGTERM/terminate to active aplay playback.
• Safety alerts still use deterministic threat scoring, cooldowns, and queue priority.
• Cloud APIs are optional or field-test-only for safety-critical paths.
• numpy==1.26.4 remains pinned for CI/Pi installs.

Validation

python -m pytest tests/test_blindnav.py tests/test_blindnav_v326.py -q

Result:

Field Notes

• Field logs used:
• Hardware tested:
• Remaining caveats:

Handoff

• Added or updated a docs/CODEX_HANDOFF_*.md note when this PR contains meaningful Codex work.