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

README.md

@@ -10,6 +10,10 @@ SaaS tutorial app, an advanced Kanban milestone sketch, and a full techdemo.
 The v1 story now has tutorial, recipe, diagnostics, and stability docs; see
 `docs/v1-readiness.md` for the remaining close-gap audit.
 
+Template authoring: `.heml` is HTML plus a small hemplate overlay for escaped
+text, trusted HTML, dynamic attributes, Rust-shaped control directives, generated
+slots/forms/handles, and keyed partial targets. See `docs/hemplate-syntax.md`.
+
 Local checkout note: until the hemplate crates are published, this repository
 expects `hemplate` checked out next to `hemx` as `../hemplate/hemplate`. The app
 scaffolder fails with that exact path if the prerequisite is missing, instead of
@@ -21,7 +25,8 @@ For beginner and production-shaped app code, stay on this path. req: public_api/
 
 1. **Templates declare the surface.** `.heml` files declare roots, slots,
    forms, handles, keys, page targets, optional pending states, and explicit
-   leaf islands with `data-hemx-*` attributes.
+   leaf islands with `data-hemx-*` attributes. The stable syntax surface lives in
+   `docs/hemplate-syntax.md`.
 2. **Build generates typed helpers.** `hemx_build::app().run()` consumes the
    hemplate Surface and emits generated Rust helpers for slots, forms, handles,
    page targets, classes, and events. req: ceremony/003 req: build/001