Add inline steering queue support

[sansari]

Summary

• queue composer submissions as inline steering while a turn is active
• preserve per-chat FIFO ordering, attachment metadata, rejection recovery, and exact-once delivery
• replay accepted undelivered steering after Stop while retaining retryable failures
• document the lifecycle and add focused unit, integration, and requirements coverage

Verification

• scripts/test-unit.sh
• npx rfc2119 check
• independent blocker and public-safety review