Refactor SmolVLA plotting to use tracker data instead of local variables

Remove local tracking variables (correction, x1_t, error) from the
denoising loop and instead retrieve plotting data from the RTC tracker
after each denoise step. This makes the code cleaner and uses the
tracker as the single source of truth for debug/visualization data.

Changes:
- Remove initialization of correction, x1_t, error before denoising loop
- After each Euler step, retrieve most recent debug step from tracker
- Extract correction, x1_t, err from debug step for plotting
- Update tracking condition to use is_debug_enabled() method

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Alexander Soare <alexander.soare159@gmail.com>

src/lerobot/policies/smolvla/modeling_smolvla.py

@@ -789,9 +789,6 @@ class VLAFlowMatching(nn.Module):
 
         x_t = noise
         time = torch.tensor(1.0, dtype=torch.float32, device=device)
-        correction = None
-        x1_t = None
-        error = None
         use_provided_axes = viz_xt_axs is not None and viz_vt_axs is not None
 
         while time >= -dt / 2:
@@ -828,9 +825,21 @@ class VLAFlowMatching(nn.Module):
             time += dt
 
             # Record x_t after Euler step (other params are recorded in rtc_processor.denoise_step)
-            if self._rtc_enabled() and correction is not None:
+            if self._rtc_enabled() and self.rtc_processor.is_debug_enabled():
                 self.rtc_processor.track(time=time, x_t=x_t)
 
+            # Retrieve data from tracker for plotting
+            correction = None
+            x1_t = None
+            error = None
+            if self._rtc_enabled() and self.rtc_processor.is_debug_enabled():
+                recent_steps = self.rtc_processor.get_recent_debug_steps(n=1)
+                if recent_steps:
+                    debug_step = recent_steps[0]
+                    correction = debug_step.correction
+                    x1_t = debug_step.x1_t
+                    error = debug_step.err
+
             # Visualize x_t using plot_waypoints - accumulate all denoise steps
             # Use provided axes or create new ones
             if not use_provided_axes: