## Why
Bounding collected output also requires limiting the raw event backlog between
V8 and the cell actor. Previously, runtime events and cell output accumulated
in an unbounded channel and vector.
## What changed
- Select the event transport and `OutputCollector` from the cell's
`output_truncation` mode. Server mode uses an ordered, bounded channel with
backpressure; client mode retains untruncated output.
- Transfer the collector with each observation and restore it on failed
delivery, preserving retained media and omission metadata.
- Include server-mode script errors in the observation's output budget. Build
completion output only after completion wins against termination, so a
rejected error cannot evict output returned by termination.
- Keep the public service on client-side truncation.
## Testing
Add regression coverage for channel backpressure, event ordering, receiver
closure, failed observation delivery, termination during completion commit,
and oversized errors across UTF-8 and omission boundaries.
GitOrigin-RevId: 7604b60416e33c636ca41138be65dc0fd452774a
Sean Huang
92e802fb61