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

AGENTS.md

@@ -56,7 +56,7 @@ Keep it stable. Prefer pointers to canonical sources over copied structure, file
 - Public examples and beginner APIs should use generated resources and `IntoEffect`, not raw ids or runtime opcodes.
 - Use `cargo run -p hemx-xtask -- app new PATH` for the generic page/form/keyed-row/notice starter, and `cargo run -p hemx-xtask -- app new --mobile PATH` for the phone-first starter with host capabilities, recovery truth, and release-kit commands. req: ceremony/005 req: ceremony/006
 - The public component-reuse explanation lives in `docs/recipes/reusable-partials.md`; do not grow a client component framework to explain partial composition.
-- Hemlate examples must use real hemplate syntax, not Vue/Handlebars sketches: `{+ expr +}` for escaped text, `{+= expr =+}` only for trusted/rendered HTML, `+attr="expr"` for dynamic attributes, and Rust-shaped `h-if`, `h-for`, `h-match`, `h-case` directives (`h-case="_"` is the default arm).
+- The stable public `.heml` authoring surface lives in `docs/hemplate-syntax.md`; Hemlate examples must use that real hemplate syntax, not Vue/Handlebars sketches.
 - JS runtime changes must preserve root-scoped lookup and avoid selectors, VDOM, expressions, and per-node listeners.
 - Host capability adapters must stay at the `hemx-host` boundary: they may call host APIs and return host events, but they must not mutate DOM or own app/domain state. req: host/002
 - Local/offline app behavior should be commands/events/projections; do not add `hemx-local`, stored DOM patches, or stored `EffectBatch` truth without a proven reusable contract. req: local/001 req: local/002