Quick Win 1: Show available boxes on "Unknown Box type" error
- vm.rs, vm_fallback.rs: Display sorted list of available user-defined boxes
- Before: "Unknown Box type: Foo"
- After: "Unknown Box type: Foo. Available: Bar, Baz, Main"
Quick Win 2: Show stderr on child process timeout
- child.rs, selfhost_exe.rs: Capture and display stderr (up to 500 chars)
- Helps diagnose what went wrong in selfhost compiler child process
Quick Win 3: Simplify Stage-B compiler API (SSOT)
- compiler_stageb.hako: Add StageBDriverBox.compile() as single entry point
- compiler_stageb.hako: Remove StageBMain compatibility wrapper
- compiler.hako: Change from `using ... as StageBMain` to direct import
- compiler.hako: Call StageBDriverBox.compile() directly
Also includes child_env.rs NYASH_MODULES env var for module mapping.
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>
