feat(diagnostics): add compiler-backed heml language service

Add hemx-lsp for stdio LSP diagnostics, completion, and hover while preserving HTML editor tooling for .heml files. Teach hemx-build to expose generated target and derive-known template context facts, including simple h-for locals, so editor help comes from build-owned facts instead of editor-only parsers. Wire VS Code/Cursor and Neovim documentation and extend the Workout exemplar with a real h-for plan loop for end-to-end proof.

req: diag/004

req: diag/005

req: diag/006

examples/workout/src/lib.rs

@@ -607,6 +607,7 @@ impl WorkoutEvent {
 
 #[derive(Hemplate)]
 pub struct Workout {
+    pub plan: Vec<ExercisePlan>,
     pub next_action: String,
     pub primary_action: String,
     pub status: String,
@@ -664,6 +665,7 @@ pub fn view(state: &WorkoutState) -> Workout {
         ),
     };
     Workout {
+        plan: state.plan.clone(),
         next_action: state.projection.next_action.clone(),
         primary_action: state.projection.primary_action.clone(),
         status: state.projection.progress.clone(),