[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.

obs/frame_scheduler.h

@@ -34,11 +34,16 @@ typedef struct LGFrameScheduler
   bool     resetPending;
   bool     immediatePending;
   bool     feedbackDirty;
+  bool     controlPending;
+  bool     controlFaulted;
   uint32_t clientID;
   uint32_t generation;
+  uint32_t controlSerial;
   uint64_t period;
   uint64_t lastSend;
   uint64_t lastTick;
+  uint64_t nextControlCheck;
+  uint64_t controlRetryDelay;
 
   int64_t  phaseError;
   uint32_t feedbackFrameSerial;