docs(design): daemon side-channel coordination v5 (fourth review round)

- Concurrent-in-session-/model drift (Critical): add §2.2 post-roundtrip
  reconciliation — on roundtrip settle the bridge re-reads the agent's actual
  model and emits a corrective model_switched if divergent (in-session /model
  bypasses modelChangeQueue, so drop-when-suppressed could otherwise leave the
  bus on A while the session runs B).
- IDE-companion lockstep (Critical): add a one-release dual-emit transition
  (publish both generic session_update and promoted approval_mode_changed) and
  enumerate the upstream dispatch sites (daemonIdeConnection.ts /
  DaemonChannelBridge.ts) that drop unknown types and also need handlers.
- Specify the model_switched payload mapping (currentModelId→data.modelId,
  envelope sessionId→data.sessionId) — without it the SDK validator drops the
  promoted event and A1 is non-functional.
- Require demux observability (structured log: promoted/dropped/suppressed/
  generic) at every decision point.
- Correct the reviewer's "replay_complete doesn't exist": it shipped in merged
  #4484 (eventBus.ts:444); A5 phase 2 introduces only session_snapshot.
- First-attach no longer synthesizes replay_complete{0} (would widen that
  event's contract); session_snapshot is self-delimiting on first attach.
- Tighten capture-at-emission to a synchronous read+publish block.
- Specify the helper-generalization migration model; resolve Q3 (keep the
  extMethod bypass); add the A4 distinguishing test (done in #4539) to §8.

Author: 秦奇