feat(build): surface structured template diagnostics

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

docs/diagnostics.md

@@ -6,7 +6,10 @@ raw ids, selector targeting, runtime opcodes, or Cargo internals in the normal
 path. req: diagnostics/001 req: diagnostics/002 req: diagnostics/003
 
 Use this guide as the v1 checklist for common mistakes in beginner and
-production-shaped apps.
+production-shaped apps. Structured `hemx-build` diagnostics expose a file path,
+directive, target, expected template fact, and repair action so an optional
+editor overlay can share compiler authority without becoming a custom editor
+framework.
 
 ## Where errors happen