Add public .heml syntax authority and refactor the unkeyed generated-target error into structured diagnostic data while preserving the human compiler error.
req: diagnostics/001
req: diagnostics/002
Document the v1 diagnostics story for common template, derive, form, result-handler, and runtime/deploy mistakes. The guide names expected user-facing diagnostics and recurring verification gates without exposing raw ids/selectors as the beginner path.
req: diagnostics/001
req: diagnostics/002
req: diagnostics/003
req: failure/004
req: failure/005
req: public_api/005
req: test/003
slhx agent