Add the colour palette: white-shade filter and palette extraction

MIL-001 tasks 1 to 6.

- Pin the toolchain and tool configuration in pyproject.toml: Python 3.13,
  colorgram.py 1.2.0, and ruff, mypy (strict) and pytest as dev tools.
- Add is_white_shade and remove_white_shades (threshold 240) with tests that
  pin the boundary at 239 and 240.
- Add extract_palette, which reads the reference image with colorgram and drops
  white shades, with tests on synthetic images and on the reference image.
- Add the reference image assets/20260524_132700.jpg and a note on it.
- Record the code review against QC-PY-001 as RC-005 (verdict Go).

Closes #1
Closes #2
Closes #3
Closes #4
Closes #5
Closes #6

Task: MIL-001#1
Task: MIL-001#2
Task: MIL-001#3
Task: MIL-001#4
Task: MIL-001#5
Task: MIL-001#6

assets/README.md

@@ -0,0 +1,5 @@
+# Assets
+
+`20260524_132700.jpg` is the reference image the colour palette is extracted
+from (JPEG, 523 by 930 pixels, 172,725 bytes, supplied by S01 on 2026-10-07).
+`REFERENCE_IMAGE_PATH` in `src/hirst_painting.py` points at it.