fix: restore cursor and SCStream capture after display sleep/wake

[cwy433-png]

Problem

After the Mac's displays sleep and wake again, the mouse cursor is no longer
visible on the tablet even though the pointer has actually moved onto the
virtual display (clicks still land). The cursor stays lost for the rest of
the session.

Root cause (two stacked issues, confirmed via /tmp/sidescreen.log)

1. When the displays go to sleep, SCStream dies with
   SCStreamErrorDomain -3815 ("no displays or windows to capture"). The
   stream-error delegate then silently switches capture to the
   CGDisplayStream fallback — and the session stays on the fallback
   forever, even after wake.
2. The fallback was created with properties: nil, and
   kCGDisplayStreamShowCursor defaults to false, so the fallback never
   composites the cursor at all.

Net effect: any sleep/wake cycle permanently drops the session onto a
capture path that cannot show the pointer.

Fix

• ScreenCapture registers NSWorkspace.screensDidWakeNotification /
  didWakeNotification observers while a capture is active (removed in
  stopStreaming(), with a defensive deinit backstop). A full system wake
  fires both notifications, so they are coalesced into a single restart.
• On wake, after a 2 s settle delay: if the fallback is active, stop it and
  retry SCStream via the existing restartStream() path (which re-enters
  the fallback on its own if SCStream still cannot start). The
  wake-triggered restart does not consume the frame monitor's one-shot
  stall-recovery budget.
• The CGDisplayStream fallback now sets CGDisplayStream.showCursor, so
  the pointer remains visible even while the fallback is in use.

One file changed (MacHost/Sources/ScreenCapture.swift), no new
dependencies, no private API.

Testing

M1 MacBook Air, macOS 15.7.5 (24G624), Android tablet over USB, HEVC,
1280x800 HiDPI. Each scenario was verified twice (before final cleanup and
on the final patch):

1. Display sleep (no password on wake) → wake: cursor visible and persists.
2. Display sleep (password required) → unlock: cursor visible and persists.
3. Full system sleep via power button → wake: cursor visible and persists.

Log-verified sequence on every cycle:
SCStream stopped with error … -3815 → CGDisplayStream fallback started
(cursor now still visible thanks to showCursor) → wake → Wake restart: leaving CGDisplayStream fallback, retrying SCStream → First frame received from SCStream (restart).

Also verified: repeated sleep/wake cycles do not stack observers or
restarts, and stopping the server unregisters the observers.

Notes — approaches tried and rejected (for reviewer context)

• Private cursor-scale nudge (CGSSetCursorScale, the programmatic
  "shake to locate"): rejected. Manual testing showed the enlarged cursor is
  visible on the tablet but the pointer disappears again the moment the
  scale returns to normal, so a transient nudge cannot fix the steady state.
  Dropping it also avoids introducing a new private API.
• CGDisplayHideCursor/CGDisplayShowCursor toggle after wake: was
  implemented behind a debug switch during testing and turned out to be
  unnecessary once the fallback/showCursor issue was fixed; removed from the
  final patch.
• Restarting the capture stream alone (without the fallback showCursor
  fix) is not sufficient — by the time the machine wakes, capture has
  already degraded to the cursor-less fallback, and a session that ever
  falls back for other reasons would still lose the pointer.
• There are reports of an OS-level cursor-compositing bug on virtual
  displays after wake (e.g. BetterDisplay discussions #999 / #2770). That
  may still exist independently on some macOS versions, but in SideScreen's
  case the dominant cause was the two issues above: on macOS 15.7.5 the
  restarted SCStream composites the cursor correctly after every wake.

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