## Why

Log ingest, artifact/file transfer, control heartbeat, and job ack/result delivery already exist as separate platform/run channels, but the current evidence mostly proves each channel in isolation. The next risk is starvation under load: a large artifact or file operation must not delay heartbeat, lifecycle acknowledgement, job result delivery, or durable log upload.

## What Changes

- Add channel-isolation requirements that define priority, bounded payloads, retry behavior, and non-starvation guarantees across run/platform channels.
- Add run-side concurrency and queue tests proving large artifact/file work cannot block control heartbeat, job ack/result submission, or log spool upload.
- Add platform service/API tests proving artifact/log/job/control endpoints preserve independent validation, state mutation, and idempotency under interleaved requests.
- Add a local verification command set that exercises platform and run test suites plus strict OpenSpec validation.
- No breaking API changes are expected; the change hardens behavior and verification around existing channel contracts.

## Capabilities

### New Capabilities
- `log-artifact-channel-isolation`: Defines cross-channel non-starvation, bounded-transfer, durable-retry, and verification guarantees for run/platform control, job, log, artifact, and file operations.

### Modified Capabilities
- None.

## Impact

- Affected roots: `run/` and `platform/`.
- Affected areas: run worker scheduling, platform client calls, log spool retry, artifact/file queue retry, platform run-facing APIs, service tests, API tests, and protocol documentation.
- Validation impact: requires focused run/platform concurrency tests, existing package tests, `scripts/check-structure.sh`, and `openspec validate harden-log-artifact-channel-isolation --strict`.