docs: image brief and treatment tooling

- docs/image-brief.md: one treatment for every image, keyed to the site palette
- tools/images/treat.py: cutout (rembg) and treat (Pillow) as separate, re-runnable steps
- tools/images/README.md: how to run it without polluting the NixOS python

tools/images/README.md

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+# Image treatment
+
+Implements `docs/image-brief.md`. Two steps, in this order.
+
+## 1. Cut the subject out
+
+Needs `rembg`. It is not installed on the host, and installing Python packages on NixOS
+into the system interpreter is a bad idea. Use a container:
+
+```
+docker run --rm -v "$PWD/raw:/in" -v "$PWD/build/cut:/out" \
+  -v "$PWD/tools/images/treat.py:/treat.py" \
+  python:3.12-slim sh -c "pip install -q rembg[cpu] pillow onnxruntime && \
+  python /treat.py cutout /in/*.jpg --out /out"
+```
+
+Or cut out by hand in GIMP and skip to step 2. For fewer than ten images, hand-cutting is
+usually better — automatic segmentation struggles with reflective or thin edges.
+
+## 2. Apply the treatment
+
+```
+pip install pillow        # or the same container, without rembg
+python3 tools/images/treat.py treat build/cut/*.png --size hero
+```
+
+Output lands in `content/assets/images/` as WebP at quality 82, canvas `#0b0d10`, subject
+right of centre, left-to-right fade, rim glow from the alpha channel.
+
+## Then
+
+1. Add the page reference: `![caption](../../assets/images/name.webp)`
+2. File the original in the project's own repo under `docs/images/`, with a note in that
+   folder's `README.md` saying where it came from.
+3. Deploy and check it on the live site at 1440×900 and on a phone.

tools/images/treat.py

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+#!/usr/bin/env python3
+"""Apply the site image treatment described in docs/image-brief.md.
+
+Two jobs, both idempotent and re-runnable:
+
+  cutout  remove the background from a product photograph      (needs rembg)
+  treat   compose onto the dark canvas with the fade and glow  (needs Pillow)
+
+Typical use:
+
+  python3 tools/images/treat.py cutout raw/*.jpg      -> build/cut/*.png
+  python3 tools/images/treat.py treat  build/cut/*.png -> content/assets/images/*.webp
+
+If rembg is not available, cut the subject out by hand in GIMP, save as PNG with
+transparency, and run only the `treat` step.
+"""
+import argparse, pathlib, sys
+
+BG        = (0x0b, 0x0d, 0x10)   # --bg
+GLOW      = (0x8e, 0xcb, 0xff)   # --link
+SIZES     = {"hero": (1600, 1000), "detail": (1200, 1200)}
+FADE_END  = 0.55                 # gradient reaches 0 opacity here
+GLOW_A    = 0.32
+GLOW_BLUR = 11
+LIFT      = 1.06                 # shadow lift on the subject
+
+
+def cutout(paths, outdir):
+    try:
+        from rembg import remove
+    except ImportError:
+        sys.exit("rembg not installed. See tools/images/README.md, or cut out by hand and skip this step.")
+    from PIL import Image
+    outdir.mkdir(parents=True, exist_ok=True)
+    for p in paths:
+        src = Image.open(p).convert("RGBA")
+        dst = remove(src)
+        target = outdir / (pathlib.Path(p).stem + ".png")
+        dst.save(target)
+        print(f"cut  {p} -> {target}")
+
+
+def treat(paths, outdir, size="hero"):
+    from PIL import Image, ImageFilter, ImageEnhance
+    W, H = SIZES[size]
+    outdir.mkdir(parents=True, exist_ok=True)
+
+    for p in paths:
+        subject = Image.open(p).convert("RGBA")
+
+        # fit to 68% of canvas height, keep aspect
+        scale = (H * 0.68) / subject.height
+        subject = subject.resize((max(1, int(subject.width * scale)), int(H * 0.68)), Image.LANCZOS)
+
+        # shadow lift
+        rgb = ImageEnhance.Brightness(subject.convert("RGB")).enhance(LIFT)
+        subject = Image.merge("RGBA", (*rgb.split(), subject.split()[3]))
+
+        canvas = Image.new("RGB", (W, H), BG)
+
+        # place right of centre
+        x = int(W * 0.52)
+        y = (H - subject.height) // 2
+        canvas.paste(subject, (x, y), subject)
+
+        # rim glow from the subject's own alpha
+        alpha = subject.split()[3].filter(ImageFilter.GaussianBlur(GLOW_BLUR))
+        glow = Image.new("RGB", subject.size, GLOW)
+        canvas.paste(glow, (x, y), alpha.point(lambda a: int(a * GLOW_A / 255)))
+
+        # left-to-right fade back to the page colour
+        fade = Image.new("L", (W, 1))
+        end = int(W * FADE_END)
+        for px in range(W):
+            fade.putpixel((px, 0), 255 if px < end else max(0, int(255 * (1 - (px - end) / max(1, W - end)))))
+        fade = fade.resize((W, H))
+        canvas = Image.composite(Image.new("RGB", (W, H), BG), canvas, fade)
+
+        target = outdir / (pathlib.Path(p).stem + ".webp")
+        canvas.save(target, "WEBP", quality=82, method=6)
+        kb = target.stat().st_size / 1024
+        print(f"treat {p} -> {target}  ({kb:.0f} KB)")
+
+
+def main():
+    ap = argparse.ArgumentParser()
+    ap.add_argument("step", choices=["cutout", "treat"])
+    ap.add_argument("paths", nargs="+", type=pathlib.Path)
+    ap.add_argument("--out", type=pathlib.Path, default=None)
+    ap.add_argument("--size", choices=list(SIZES), default="hero")
+    a = ap.parse_args()
+    out = a.out or pathlib.Path("build" if a.step == "cutout" else "content/assets/images")
+    (cutout if a.step == "cutout" else treat)(a.paths, out) if a.step == "cutout" else treat(a.paths, out, a.size)
+
+
+if __name__ == "__main__":
+    main()