Streaming stack is on-demand, not always-on

Measured rather than assumed: the compositor and Sunshine together hold
220 MiB on the 3060 — sway ~112, Sunshine ~108. That card also runs the
local LLM, and with an 11.3 GB model resident on a 12 GB card the free
headroom goes from 992 MiB to 772. Losing a fifth of what is left matters
for KV cache growth, so leaving it running was the wrong default.

sway-headless and sunshine no longer start at boot. `stream on` brings them
up, `stream status` reports state and VRAM, and sunshine-idle-stop.timer
checks every five minutes whether any Moonlight client still holds a
connection to 47984/47989/48010 — stopping the stack after 15 idle minutes
so a forgotten session does not sit on memory the inference stack wants.

Note for future edits: `systemctl --user disable` on a unit dotbot has
symlinked into ~/.config/systemd/user deletes the symlink itself, because
systemd treats it as a linked unit. Re-run ./install after disabling one.

hosts/mainframe/systemd/sunshine-idle-stop.timer

@@ -0,0 +1,11 @@
+[Unit]
+Description=Check every 5 minutes whether the streaming stack can be shut down
+ConditionHost=mainframe
+
+[Timer]
+OnBootSec=5min
+OnUnitActiveSec=5min
+AccuracySec=30s
+
+[Install]
+WantedBy=timers.target