Start RC viewing without operator device enrollment

[AetherAI3]

Problem

aether rc start required aether device enroll, but that enrollment is operator-only. Ordinary accounts could have RC viewing enabled and still be unable to start an observer session.

Change

• Persist a stable local installation ID and ask Cloud's owner-scoped POST /remote/device-identity endpoint for the RC-only device label.
• Use the returned label for session registration, host attach, and presence. RC no longer reads the SC-DEVICE enrollment record on its default path.
• Fail before registration when Cloud refuses or cannot verify the RC identity. A corrupt local installation ID is not silently replaced.
• Update remote-viewing guidance while retaining the separate live ordinary-account qualification requirement.

The Cloud endpoint is in AETHER-CLOUD #1944, which should merge before this PR is released. The RC label is not a control credential; existing control paths remain separately authenticated.

Verification

• npm run build passed.
• node --test --test-isolation=none dist/test/rc_device_identity.test.js dist/test/rc_command.test.js dist/test/rc_lifecycle.test.js dist/test/rc_viewer_host.test.js: 114 passed, 0 failed, 0 skipped.
• The normal-account fixture starts and publishes an RC session with zero /device/v1/ calls. Other tests cover stable local identity, corrupt-state refusal, and a 403 that stops before session registration.
• git diff --cached --check passed.

Live deployment qualification remains separate from these source tests.

Closes #300