v0.1.0b4

Changed

• The policy-flow diagram in the README is now a pre-rendered D2 SVG,
  not a Mermaid code block. PyPI's readme-renderer does not support
  Mermaid extensions, so v0.1.0b3's project page rendered the diagram
  as raw Mermaid source. The new layout keeps the diagram as
  diagram-as-code (assets/diagrams/policy-flow.d2) but commits the
  rendered output (policy-flow.svg) alongside, and the README
  references it via a raw GitHub URL on the repo's default branch.
  Result: the image renders identically on GitHub, on PyPI, and on
  every other Markdown viewer.

Tooling

• New nox -s diagrams session and make diagrams shim render
  assets/diagrams/*.d2 to SVG via the d2 binary. Same opt-in
  skip-with-warning pattern as markdown_lint and actions_lint —
  contributors without d2 installed get a clean run with a clear
  pointer to install instructions (Gentoo overlay / d2lang.com).