🎨 assets: add a logo, a frozen dict

[nstarman]

xmmutablemap had no logo. This gives it one: a snowflake between a dict's braces, for a mapping that can't change. It uses GalacticDynamics' purple and teal.

docs/_static/make_logo.py writes it:

uv run docs/_static/make_logo.py                     # docs/_static/favicon.svg
uv run docs/_static/make_logo.py --size 2048 big.png # a PNG, at any size

• SVG by default: 943 bytes and sharp at any size. The script writes the SVG text directly, so the default needs no dependencies.
• Geometry: the braces and the snowflake (its arms, and where its side branches grow and how long they are) are constants at the top of the script.
• PNG on request: rendered with resvg-py, declared in the script's PEP 723 header, so uv run fetches it.
• Location: docs/_static/, as in the other GalacticDynamics packages, ready for when xmmutablemap has docs. Nothing references it yet.

Checked:

• The pre-commit hooks pass on the new files. The mypy hook is limited to src/, so it doesn't apply.
• Re-running the script reproduces the file, and it reads on light and dark backgrounds and at 32 px.

Part of giving the GalacticDynamics packages scripted logos.

🤖 Generated with Claude Code

[github-code-quality]

Code Coverage Overview

Languages: Python

Python / code-coverage-agent

The overall line coverage in commit 2cc880f in the docs/logo branch remains at 100%, unchanged from commit de309bb in the main branch.