Support exact, dev, and custom Zig toolchains in corpus manifests

Problem

Large Zig projects often require exact, dev, or custom Zig versions. The current stable-lane model is not enough for projects such as ZLS and Mach.

Needed

• Represent exact stable versions, dev build windows, and custom distributions.
• Resolve toolchains from configured paths, local cache, official Zig downloads, and known mirrors.
• Allow repo manifests to declare a required toolchain separately from Azazel's own supported generator lanes.
• Emit precise diagnostics when the requested toolchain is unavailable or too new/old.

Corpus drivers

• ZLS requires an older accepted 0.17-dev window and rejects current master.
• Mach requires 2026.4.10-mach.
• Capy requires at least 0.14.1 but still exposes transitive dependency drift.

Done when

• tools/huge_corpus.py --build --repo zls reports the exact missing/unsupported dev-toolchain window.
• tools/huge_corpus.py --build --repo mach can use a locally installed Mach Zig or reports a mirror/toolchain failure distinctly.
• Docs explain stable lanes vs project-required toolchains.

Start with tools/huge_corpus.py and trace how corpus builds, repo manifests, and current stable generator lanes select Zig toolchains. Use the ZLS, Mach, and Capy corpus drivers to understand the required cases. Done means the stated build commands distinguish unavailable, unsupported, and mirror/toolchain failures, while the documentation explains stable lanes versus project-required toolchains.