Add first-class system dependency and pkg-config diagnostics

[godofecht]

Problem

System dependencies are currently external failures. Replacement tooling should make them explicit before build execution.

Needed

• Model pkg-config libraries, required tools, frameworks, system libraries, include paths, and headers.
• Add preflight diagnostics for missing tools/libs.
• Keep platform-specific requirements in manifests.

Corpus drivers

• River requires wayland-scanner and Wayland/wlroots related packages.
• libxev has platform-specific system integration.
• zig-gamedev uses Apple frameworks and C++ linkage on macOS.

Done when

• tools/huge_corpus.py --build --repo river reports missing wayland-scanner as a first-class requirement.
• Azazel docs show how to declare host tools and pkg-config requirements.

[godofecht]

Progress in #23:

• Added tools/huge_corpus.py --doctor.
• Manifests can now declare required host tools and pkg-config probes.
• River now reports missing wayland-scanner as a local prerequisite before build proof.

Remaining work:

• Promote this from corpus metadata into first-class Azazel project schema/runtime diagnostics.
• Add platform-specific system dependency declarations to project.cue.