WIP: 0.5.0 diagnostics

[funwithcthulhu]

WIP for 0.5.0 environment diagnostics.

Current work:

• warn when macOS Homebrew gcc/binutils commands appear to shadow Xcode tools;
• warn when Apple Silicon shells appear to mix native and Rosetta Homebrew prefixes;
• warn when installed packages declare opam build-env metadata;
• warn when known legacy compatibility packages are installed in the active switch;
• keep checks read-only and covered by fake process/env tests.

This remains draft while the 0.5 behavior gets review and any follow-up tightening.