[obs] scheduler: serialize cadence controls

Track the LGMP serial for each accepted frame-schedule message and wait
for its acknowledgement before sending another. Preserve pending RESET,
IMMEDIATE, and feedback state after definite enqueue failures.

Use bounded retry polling and request one fresh immediate frame after an
ambiguous pointer-queue fault recovers.

client/src/main.c

@@ -133,6 +133,9 @@ static void lgInit(void)
   g_cursor.inView        = false;
   g_cursor.viewReq       = false;
   g_cursor.exit          = false;
+  g_cursor.exitWait      = false;
+  g_cursor.exitRetry     = false;
+  g_cursor.exitDelta     = (struct DoublePoint) { 0 };
   g_cursor.guest.valid   = false;
 
   // if spice is not in use, hide the local cursor