Array: read out the depth buffer; show it in the readout §05

igc_array.py gains readout_depth() -> the 24-bit Z stored in pixel memory as a
depth image (near=bright), i.e. the plane the decoded SENDE z-sweep interpolates.
Readout §05 now shows the depth buffer next to the tile footprint, ties the
z-decode to a visible array output, and reframes the "remaining work" as numeric
reconstruction across regions (the coefficients are already cross-validated).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

emulator/firmware-decomp/igc_array.py

@@ -161,6 +161,44 @@ class IGCArray:
                 _wword(pix, G_BIT0, C_BITS, gv)
                 _wword(pix, B_BIT0, C_BITS, bv)
 
+    def readout_depth(self, bg=(7, 11, 17)):
+        """Read the 24-bit z stored in pixel memory back into a depth image (near=bright).
+        This is the plane the SENDE z-sweep interpolates, straight out of pixel memory."""
+        img = bytearray(self.W * self.H * 3)
+        for y in range(self.H):
+            o = y * self.W * 3
+            for x in range(self.W):
+                img[o] = bg[0]; img[o + 1] = bg[1]; img[o + 2] = bg[2]; o += 3
+        # gather the touched z range for contrast
+        zmin, zmax = Z_FAR, 0
+        for t in self.tiles.values():
+            if not t.touched:
+                continue
+            for pix in t.mem:
+                z = _rword(pix, Z_BIT0, Z_BITS)
+                if z != Z_FAR:
+                    zmin = min(zmin, z); zmax = max(zmax, z)
+        span = (zmax - zmin) or 1
+        for (tx, ty), t in self.tiles.items():
+            if not t.touched:
+                continue
+            for ly in range(TILE_Y):
+                gy = t.y0 + ly
+                if gy >= self.H:
+                    break
+                base = ly << TILE_X_BITS
+                for lx in range(TILE_X):
+                    gx = t.x0 + lx
+                    if gx >= self.W:
+                        break
+                    z = _rword(t.mem[base + lx], Z_BIT0, Z_BITS)
+                    if z == Z_FAR:
+                        continue
+                    v = 1.0 - (z - zmin) / span          # near = bright
+                    o = (gy * self.W + gx) * 3
+                    img[o] = int(30 + v * 120); img[o + 1] = int(90 + v * 150); img[o + 2] = int(120 + v * 130)
+        return img
+
     def readout(self, bg=(7, 11, 17)):
         """Read pixel memory back out into an RGB framebuffer (row-major, RGB bytes)."""
         img = bytearray(self.W * self.H * 3)