[T1] Peek stream isolation

[dennisonbertram]

[parent] #1339

Problem

Peek events indistinguishable from main/conversation stream events (model.go ~4496): dashboard peek can route foreign messages into transcript and freeze live rendering.

Outcome

Wrap peek messages in distinct msg type carrying channel identity; route only those to peek; always re-arm originating stream's poll.

Acceptance criteria

• Peek active + live run → transcript keeps rendering; resumes after close.

Test-first plan

Peek-vs-live rendering regression test (expected red). HIGHEST priority.

Impact analysis

TUI-only.