CLAUDE: renderNadirSequence(): add -NADIR-SCENE-PXPYD-NULLCAM debug dump

Second per-scene grid computed with scene_QuadClt=null - the exact camera
model used inside Cuas.transformFromVirtual()'s refinement loop. Its
non-uniformity isolates the refinement residual; its difference from
-NADIR-SCENE-PXPYD isolates the scene-ERS-instance effect. Saved only on
the first (non-undistorted) pass.

Context: 2026-07-10 analysis of the first -NADIR-SCENE-PXPYD dump showed
the sampling grid contains rotation ~0.31x scene roll (R2=0.77) plus
correlated scale (~0.955) and shift_y (to -24 px) concentrated in the
high-roll-rate half of the graveyard segment - explaining the ~0.6x
in-plane content rotation and COLMAP's f=1000/k1=-0.38 self-calibration.
Co-authored-by: Claude <claude@elphel.com>

src/main/java/com/elphel/imagej/tileprocessor/OpticalFlow.java

@@ -10955,6 +10955,12 @@ java.lang.NullPointerException
     	// Debug: collect the actual per-scene sampling grids fed to renderGPUFromDSI
     	// (scene-frame pXpYD after transformToScenePxPyD and optional undistort).
     	double[][][] dbg_scene_pXpYD = new double[selected_scenes.length][][];
+    	// Debug: same transform but with scene_QuadClt=null - the exact camera model
+    	// used inside Cuas.transformFromVirtual()'s refinement loop.  Non-uniformity
+    	// of this grid = pure refinement residual; difference from dbg_scene_pXpYD =
+    	// pure effect of the scene's own ERS instance. Saved only on the first
+    	// (non-undistorted) pass.
+    	double[][][] dbg_nullcam_pXpYD = undistort ? null : new double[selected_scenes.length][][];
     	ImageStack stack_scenes = null;
     	for (int nscene = 0; nscene < selected_scenes.length; nscene++) {
     		if (nadir_pXpYD[nscene] == null) continue;
@@ -10985,6 +10991,14 @@ java.lang.NullPointerException
     			undistortPxPy(pXpYD_scene, selected_scenes[nscene].getErsCorrection());
     		}
     		dbg_scene_pXpYD[nscene] = pXpYD_scene;
+    		if (dbg_nullcam_pXpYD != null) {
+    			dbg_nullcam_pXpYD[nscene] = transformToScenePxPyD(
+    					nadir_pXpYD[nscene], // final double [][] reference_pXpYD,
+    					scene_xyz,           // final double []   scene_xyz,
+    					scene_atr,           // final double []   scene_atr,
+    					ref_scene,           // final QuadCLT     reference_QuadClt,
+    					null);               // final QuadCLT     scene_QuadClt (as in transformFromVirtual refinement)
+    		}
     		// Render from scene's own position: ZERO3 offset, scene is its own reference
     		ImagePlus imp_scene = QuadCLT.renderGPUFromDSI(
     				-1,                               // final int       sensor_mask (-1 = merge all)
@@ -11041,6 +11055,22 @@ java.lang.NullPointerException
     					null,             // String    suffix,
     					imp_scene_pxpyd); // ImagePlus imp
     		}
+    		if (dbg_nullcam_pXpYD != null) {
+    			double [][][] nullcam_maps = ShowDoubleFloatArrays.transposeJagged(
+    					dbg_nullcam_pXpYD, 3, tilesX * tilesY);
+    			ImagePlus imp_nullcam = ShowDoubleFloatArrays.showArraysHyperstack(
+    					nullcam_maps,                                    // double [][][]  pixels,
+    					tilesX,                                          // int            width,
+    					ref_scene.getImageName() + "-NADIR-SCENE-PXPYD-NULLCAM", // String title,
+    					slice_titles,                                    // String []      titles,
+    					frame_titles,                                    // String []      frame_titles,
+    					false);                                          // boolean        show
+    			if (imp_nullcam != null) {
+    				ref_scene.saveImagePlusInModelDirectory(
+    						null,          // String    suffix,
+    						imp_nullcam);  // ImagePlus imp
+    			}
+    		}
     	}
     	if (stack_scenes == null) return null;
     	ImagePlus imp_result = new ImagePlus(suffix, stack_scenes);