Repository focus: extract Danzig into a standalone dogfood project

Problem

src/danzig, its VST3 documentation, examples, and quickstart are useful integration pressure for Azazel, but a plugin framework is not part of the build-system product surface. Keeping it in the root repository makes the package and documentation look broader and less focused than the actual production contract.

PR #43 explicitly classifies Danzig as dogfood rather than Azazel API. The next step is physical separation.

Goal

Move Danzig to a standalone repository that consumes Azazel as a real external project. Keep a small integration reference or CI link here if useful, but do not ship Danzig source as part of the Azazel package.

Acceptance criteria

• Danzig has its own repository/history and build instructions.
• The standalone Danzig project builds through a pinned Azazel version/revision.
• src/danzig, Danzig examples, and Danzig-specific root documentation are removed from the Azazel package surface.
• Azazel CI retains an external dogfood/integration check or corpus entry so Danzig still pressure-tests the build model.
• Root project.cue is a build-system fixture rather than a mixed build-system/plugin-framework project.

This should be coordinated with #45 so the extracted project proves the centrally versioned adoption path.

Start by reading PR #43 and coordinating with issue #45, then inspect src/danzig, the Danzig examples and VST3 documentation, root documentation, project.cue, and the existing CI configuration. Map what must move into the standalone project and what integration reference remains; done means the external project builds against a pinned Azazel revision and the Azazel package no longer ships Danzig source while retaining an integration check.