composite: replace bilateral-filter sky with atmospheric halo on dark background

The previous approach cropped the sky-cam frame tightly around the moon and
upscaled it as the composite background, causing cloud/haze texture to blow up
into a "wrinkled silver sheet" behind the moon disk.

New _atmospheric_sky_background() applies a radial blend:
- Inside 1.3× the moon radius the real sky is gamma-darkened (γ=1.6, ×0.65)
  so diffuse cloud texture collapses toward black while the bright atmospheric
  halo survives nearly intact.
- Beyond 2.2× the radius it transitions smoothly to a near-black night colour
  (R4 G6 B14), ensuring frame corners are properly dark rather than silver-grey.

Also removes the cv2 bilateral-filter dependency (only usage in the file).

https://claude.ai/code/session_01HuJ83KvMvshiY6HxJbtMsc

moon_composite.py

@@ -104,6 +104,44 @@ def _square_crop_to_disk(ref: Image.Image, threshold: int = 25) -> Image.Image:
     return ref.crop((left, top, right, bottom))
 
 
+def _atmospheric_sky_background(
+    sky_img: Image.Image,
+    moon_cx: float,
+    moon_cy: float,
+    moon_diam_px: float,
+) -> Image.Image:
+    """Real atmospheric halo on a deep night-sky backdrop.
+
+    Inside ~1.8× the moon radius the real sky is preserved (gamma-darkened to
+    collapse diffuse cloud texture while the bright atmospheric glow survives).
+    Beyond ~4.5× the radius it fades smoothly to a near-black night colour, so
+    the result looks like a genuine dark-sky photograph rather than an
+    over-exposed, heavily upscaled camera frame.
+    """
+    arr = np.asarray(sky_img).astype(np.float32)
+    h, w = arr.shape[:2]
+
+    # Deep night-sky colour — very dark desaturated blue
+    night = np.array([4, 6, 14], dtype=np.float32)
+    dark_bg = np.broadcast_to(night, (h, w, 3)).copy()
+
+    yy, xx = np.ogrid[0:h, 0:w]
+    dist = np.sqrt((xx - moon_cx) ** 2 + (yy - moon_cy) ** 2)
+
+    r = moon_diam_px / 2.0
+    inner_r = 1.3 * r   # real-sky halo fully preserved inside here
+    outer_r = 2.2 * r   # fully dark outside here — keeps frame corners black
+
+    alpha = np.clip((outer_r - dist) / (outer_r - inner_r), 0.0, 1.0)[..., None]
+
+    # Gamma + scale: compresses diffuse cloud texture toward black while the
+    # bright atmospheric halo (already near-white) survives nearly intact.
+    real_darkened = np.power(np.clip(arr / 255.0, 0, 1), 1.6) * 0.65 * 255.0
+
+    blended = alpha * real_darkened + (1.0 - alpha) * dark_bg
+    return Image.fromarray(np.clip(blended, 0, 255).astype(np.uint8))
+
+
 def _disk_mask(size: int, feather_px: int = 6) -> Image.Image:
     """Soft circular alpha mask the size of the reference moon image."""
     m = Image.new('L', (size, size), 0)
@@ -191,18 +229,17 @@ def composite_full_moon(
     crop = src.crop((left, top, left + crop_w, top + crop_h))
     bg = crop.resize(output_size, LANCZOS)
 
-    # ── Smooth upscaling artifacts in the sky background ──
-    import cv2
-    bg_arr = cv2.bilateralFilter(np.array(bg), d=9, sigmaColor=75, sigmaSpace=75)
-    bg = Image.fromarray(bg_arr)
-
-    # ── Where is the moon's center within the upscaled background? ──
+    # ── Moon centre in output space ──
     moon_x_in_crop = cx - left
     moon_y_in_crop = cy - top
     scale = output_size[1] / crop_h
     out_moon_cx = moon_x_in_crop * scale
     out_moon_cy = moon_y_in_crop * scale
 
+    # ── Replace raw upscaled sky with atmospheric halo on dark background ──
+    out_moon_diam = output_size[1] * moon_height_pct
+    bg = _atmospheric_sky_background(bg, out_moon_cx, out_moon_cy, out_moon_diam)
+
     # ── Load reference texture, tight-crop to disk ──
     ref = Image.open(ref_moon_path).convert('RGB')
     ref = _square_crop_to_disk(ref)