Production adoption: make Azazel a centrally versioned dependency

[godofecht]

Problem

The current adoption path still encourages projects to copy Azazel's build.zig, schema, exporter, and generator files into the project. That weakens one of Azazel's strongest reasons to exist: absorbing std.Build API churn once in Azazel instead of redistributing that maintenance burden into every consumer repository.

Goal

Define an installation/bootstrap shape where a consuming project pins one Azazel release/revision and keeps only its project-specific model locally.

A normal upgrade should change the Azazel pin, not require manually copying a new executor/generator bundle into every project.

Acceptance criteria

• A fresh external Zig project can pin Azazel through a documented versioned mechanism.
• The consuming repo owns its project.cue and project-specific export/config, but the compatibility engine is sourced from the pinned Azazel version.
• azazel check, azazel gen, and azazel build use the pinned implementation rather than a copied stale CLI/generator.
• Upgrading the pin is sufficient to pick up a future std.Build compatibility fix.
• A minimal external fixture is built in CI from a clean checkout to prove the adoption path.
• Documentation distinguishes vendoring as an explicit fallback rather than the primary installation model.