Drift-compensating frame throttle, fix FPS startup spike

Replace per-frame sleep(remaining) with absolute next_frame_time
tracking so asyncio.sleep() overshoots are recovered in subsequent
frames, keeping average FPS on target. Skip first FPS sample to
avoid ~2000+ spike from near-zero init interval.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/wled_controller/core/processing/wled_target_processor.py

@@ -427,6 +427,8 @@ class WledTargetProcessor(TargetProcessor):
             f"(display={self._resolved_display_index}, fps={self._target_fps})"
         )
 
+        next_frame_time = time.perf_counter()
+
         try:
             with high_resolution_timer():
                 while self._is_running:
@@ -516,9 +518,10 @@ class WledTargetProcessor(TargetProcessor):
                                 f"({len(send_colors)} LEDs) — send={send_ms:.1f}ms"
                             )
 
-                        # FPS tracking
+                        # FPS tracking (skip first sample — interval from loop init is near-zero)
                         interval = now - prev_frame_time_stamp
                         prev_frame_time_stamp = now
+                        if self._metrics.frames_processed > 1:
                             fps_samples.append(1.0 / interval if interval > 0 else 0)
                             self._metrics.fps_actual = sum(fps_samples) / len(fps_samples)
 
@@ -534,19 +537,24 @@ class WledTargetProcessor(TargetProcessor):
                         self._metrics.last_error = str(e)
                         logger.error(f"Processing error for target {self._target_id}: {e}", exc_info=True)
 
-                    # Throttle to target FPS
-                    elapsed = now - loop_start
-                    remaining = frame_time - elapsed
-                    if remaining > 0:
+                    # Drift-compensating throttle: sleep until the absolute
+                    # next_frame_time so overshoots in one frame are recovered
+                    # in the next, keeping average FPS on target.
+                    next_frame_time += frame_time
+                    sleep_time = next_frame_time - time.perf_counter()
+                    if sleep_time > 0:
                         t_sleep_start = time.perf_counter()
-                        await asyncio.sleep(remaining)
+                        await asyncio.sleep(sleep_time)
                         t_sleep_end = time.perf_counter()
                         actual_sleep = (t_sleep_end - t_sleep_start) * 1000
-                        requested_sleep = remaining * 1000
+                        requested_sleep = sleep_time * 1000
                         jitter = actual_sleep - requested_sleep
                         _diag_sleep_jitters.append((requested_sleep, actual_sleep))
                         if jitter > 10.0:  # >10ms overshoot
                             _diag_slow_iters.append(((t_sleep_end - loop_start) * 1000, "sleep_jitter"))
+                    elif sleep_time < -frame_time:
+                        # Too far behind — reset to avoid burst catch-up
+                        next_frame_time = time.perf_counter()
 
                     # Track total iteration time
                     iter_end = time.perf_counter()