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

[godofecht]

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.