[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.h

@@ -301,9 +301,18 @@ struct CursorState
   /* true if a pointer exit is waiting for confinement to release */
   bool exit;
 
+  /* true if a surface exit is waiting for a pointer leave */
+  bool exitWait;
+
+  /* true if confinement is being restored after a failed exit */
+  bool exitRetry;
+
   /* the local pointer exit target */
   struct DoublePoint exitPos;
 
+  /* input retained while confinement is restored */
+  struct DoublePoint exitDelta;
+
   /* true if the guest should be realigned to the host when next drawn */
   bool realign;