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(),

examples/workout/templates/workout.heml

@@ -6,6 +6,16 @@
     <p class="session-status" data-hemx-slot="status">{+ self.status +}</p>
   </section>
 
+  <section class="plan-card" aria-labelledby="plan-heading">
+    <h2 id="plan-heading">Today&apos;s plan</h2>
+    <ul>
+      <li h-for="exercise in &self.plan" h-key="exercise.name">
+        <span>{+ exercise.name +}</span>
+        <span>{+ exercise.target_sets +} × {+ exercise.reps +} @ {+ exercise.kg +}kg</span>
+      </li>
+    </ul>
+  </section>
+
   <section class="action-rail" aria-label="Current workout action">
     <button class="primary-action" type="button" data-hemx-handle="complete_set">
       <span data-hemx-slot="primary_action">{+ self.primary_action +}</span>