Roadmap: make Azazel credible for large Zig build graph replacement

Goal

Make Azazel a credible replacement layer for large real Zig build graphs, not just a declarative toy for small examples.

Current evidence

The huge corpus runner has real build proof for:

• libxev: builds on Zig 0.16.0
• libvaxis: builds on Zig 0.16.0
• zig-gamedev: builds on Zig 0.15.2

The remaining corpus repos are blocked by concrete non-Azazel or missing-feature issues:

• zls: supported Zig dev-toolchain window
• river: host pkg-config / wayland-scanner
• mach: custom Mach Zig toolchain
• microzig: dependency fetch/temp zip failure
• capy: transitive package format drift

Roadmap

• Resolve exact/dev/custom Zig toolchains.
• Model package dependencies from build.zig.zon well enough to diagnose/fetch/replay them.
• Model generated files and run-artifact outputs.
• Model nested workspaces and subproject build packages.
• Add executable parity commands for declared target slices.
• Produce artifact-level proof: built target exists, expected generated outputs exist, tests run when available.

Done when

• At least three corpus repos have an Azazel target slice that builds through Azazel, not only upstream zig build.
• tools/huge_corpus.py --build --parity produces machine-readable results with no false claims.
• Documentation clearly distinguishes upstream-build proof from Azazel replacement parity.

Start with tools/huge_corpus.py and its --build --parity path, using the listed corpus results as the baseline. Review how build.zig.zon dependencies, generated files, run-artifact outputs, and nested workspaces are represented. Done means at least three corpus repositories build through Azazel and the parity command emits machine-readable results without false claims.