feat(host/session-watch): default Gaming↔Desktop follow on for Bazzite/SteamOS

The mid-stream session watcher (rebuild the backend in place when the box
flips Gaming↔Desktop) was opt-in via PUNKTFUNK_SESSION_WATCH, so it never
ran on a stock Bazzite/SteamOS box — switching modes froze the stream on the
now-dead compositor. Default it ON when os-release ID/ID_LIKE is
bazzite/steamos (the platforms that flip sessions); still off on plain
desktops. Also parse the env properly so PUNKTFUNK_SESSION_WATCH=0 actually
disables it (was: any value, including "0", enabled it).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packaging/bazzite/host.env

@@ -27,5 +27,6 @@ PUNKTFUNK_ZEROCOPY=1
 #   PUNKTFUNK_GAMESCOPE_ATTACH=1
 #   PUNKTFUNK_GAMESCOPE_APP=steam -gamepadui   # only for an ad-hoc bare-spawn fallback
 #
-# Follow a Gaming<->Desktop switch MID-STREAM (rebuild the backend in place, no reconnect):
-#   PUNKTFUNK_SESSION_WATCH=1
+# Follow a Gaming<->Desktop switch MID-STREAM (rebuild the backend in place, no reconnect). This is
+# ON BY DEFAULT on Bazzite/SteamOS (the host detects the platform); set =0 to disable it:
+#   PUNKTFUNK_SESSION_WATCH=0