Remote sessions gain a still-here check for silent streams

What

New code helps remote and cloud sessions tell a quiet connection from a dead one:

• A silence watcher probes the session after about 8 seconds without traffic (silentBeforeProbeMs: 8000) and can give up on the stream after repeated missed probes.
• The session client now fires an onStreamSilent callback when its liveness timer runs out.
• The remote-tools announcer gains a sayStillHere method that posts a rate-limited announce carrying the capability says_still_here. A failed post is handled separately.
• The receiving side accepts a still-here ping only when the host lists that capability, and only from the client that holds the machine. Otherwise it answers invalid_announce: only the client that holds a machine can say it is still here. Accepted pings are noted in the machine registry.
• The watcher runs only when the host advertises says_still_here, and the announce handling also depends on stillHereOn().
• New still-here constants and a still-here notice are added to the remote-control event list.

The watcher is tied to the tengu_violin_pernambuco flag. The flag server returned off for this site's account and for an anonymous baseline, but no reading has been taken under this release.

Why

If enabled, a remote session whose stream silently stops could be detected and recovered instead of hanging. One finding describes it as machinery with no direct user effect.