Python/OpenCV proof of concept plus first native Swift port

Snapshot before the Python POC is removed. The POC (main.py, effects.py,
panel.py, shaders.py, effects/*.glsl, flake.nix) is the reference for the
native app under native/.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

panel.py

@@ -0,0 +1,61 @@
+"""Control panel drawn straight onto the output canvas: click a layer to toggle it, drag its sliders."""
+import cv2
+
+BG = (26, 24, 22)
+TRACK = (52, 48, 46)
+FG = (228, 228, 228)
+DIM = (125, 125, 125)
+ACCENT = (255, 176, 64)
+FONT = cv2.FONT_HERSHEY_SIMPLEX
+
+
+class Panel:
+    WIDTH = 260
+
+    def __init__(self, layers):
+        self.layers = layers
+        self.hits = []  # (x0, y0, x1, y1, target, bar_x0, bar_x1), rebuilt on every draw
+        self.drag = None
+
+    def draw(self, canvas, x0, status):
+        h = canvas.shape[0]
+        canvas[:, x0:] = BG
+        self.hits = []
+        x, right = x0 + 14, x0 + self.WIDTH - 14
+        cv2.putText(canvas, "MOTIONLAB", (x, 26), FONT, 0.55, FG, 1, cv2.LINE_AA)
+        cv2.putText(canvas, status, (x + 124, 26), FONT, 0.4, DIM, 1, cv2.LINE_AA)
+
+        y = 44
+        for layer in self.layers:
+            on = layer.enabled
+            cv2.rectangle(canvas, (x, y), (x + 12, y + 12), ACCENT if on else DIM, -1 if on else 1)
+            cv2.putText(canvas, layer.name, (x + 22, y + 11), FONT, 0.48, FG if on else DIM, 1, cv2.LINE_AA)
+            self.hits.append((x0, y - 5, x0 + self.WIDTH, y + 17, layer, 0, 0))
+            y += 22
+            if not on:
+                continue
+            bar = x + 100
+            for name, p in layer.params.items():
+                cv2.putText(canvas, name, (x + 4, y + 8), FONT, 0.36, DIM, 1, cv2.LINE_AA)
+                cv2.rectangle(canvas, (bar, y), (right, y + 8), TRACK, -1)
+                cv2.rectangle(canvas, (bar, y), (bar + int((right - bar) * p.norm), y + 8), ACCENT, -1)
+                self.hits.append((bar - 8, y - 4, right + 8, y + 12, p, bar, right))
+                y += 16
+            y += 4
+
+        cv2.putText(canvas, "[h] panel  [s] shot  [r] rec  [q] quit", (x, h - 12), FONT, 0.36, DIM, 1, cv2.LINE_AA)
+
+    def on_mouse(self, event, mx, my, flags, _):
+        if event == cv2.EVENT_LBUTTONDOWN:
+            for x0, y0, x1, y1, target, bar_x0, bar_x1 in self.hits:
+                if x0 <= mx < x1 and y0 <= my < y1:
+                    if bar_x1:
+                        self.drag = (target, bar_x0, bar_x1)
+                    else:
+                        target.enabled = not target.enabled
+                    break
+        elif event == cv2.EVENT_LBUTTONUP:
+            self.drag = None
+        if self.drag and event in (cv2.EVENT_LBUTTONDOWN, cv2.EVENT_MOUSEMOVE):
+            param, bar_x0, bar_x1 = self.drag
+            param.norm = (mx - bar_x0) / (bar_x1 - bar_x0)