Spec 11 acceptance pass: switch handover feel, slow-pick cover, two-phase off [spec 11 §5-§6]

[wine-fall]

What it is

Spec 11 (agentic steer) is code-complete and unit/smoke-verified, but its
acceptance is partly sensory and only the listener can run it. Owed, on the
real radio (make dev, music on):

1. The motivating script: during a song, say "换一首歌" — the reply should
   acknowledge and cover the wait over the still-playing track, the air should
   hand over to a different track when the pick lands (no dead air, old track
   never plays to its natural end), and the new track should introduce itself
   only at handover. Watch .dev/dev.log for music.switch due → pick stages
   → music.switch handover.
2. Handover feel: the swap is currently a hard cut (engine is
   single-music). Judge whether it needs a short fade (~0.5s) — spec 11 §6
   holds this open question.
3. Slow-pick cover: if a pick runs long (>~60s), is the ducked-music wait
   enough, or should the radio volunteer a "still looking" line? Spec 11 §6 —
   decide by ear; no mechanism reserved.
4. Two-phase off: "帮我关掉" must ask for confirmation and keep playing; a
   confirm must sign off and close as cleanly as /quit; "好困啊" must do
   nothing.
5. Specific request: name a style/artist ("来点爵士") and judge whether the
   handed-over track honors it.

Spec

specs/spec11/11-agentic-steer.md §5 (acceptance criteria 1-4), §6 (the two
open by-ear questions).

Done when

The listener has run the five checks above and given a verdict. A "no" on any
item becomes its own follow-up issue (feel items may also resolve spec 11 §6's
open questions — record the decision in the spec); this issue closes on the
pass having been run, not on everything being perfect.