OSDUIHelper jetsam-exits when idle and launchd spawns a fresh PID on
the next media-key press, so the one-shot SIGSTOP only suppressed the
first incarnation. SystemOSDManager now polls for the PID after
kickstart and runs a background watcher that re-SIGSTOPs every new
incarnation, with state guarded by OSAllocatedUnfairLock. Watcher is
torn down on enableSystemHUD().
Fixes the legacy OSDUIHelper duplicate-HUD path in #419.
The macOS 26 Liquid Glass HUD (loginwindow -> BezelServices ->
ControlCenter SystemBannerService) is a separate render path gated by
the com.apple.private.system-banner-client entitlement and is not
suppressible from a regular user-space app; it needs a privileged
SMAppService helper and is out of scope here.
Signed-off-by: Jis G Jacob <studiokeys@blissroms.org>