fix(build): explain missing keyed loop facts

Improve the unkeyed h-for diagnostic so authors see the exact loop, stable h-key shape, generated helper consequence, and why dynamic child data-key is not enough.

req: scope/001

req: diagnostics/002

hemx-build/src/lib.rs

@@ -1543,11 +1543,16 @@ fn reject_unkeyed_loop(
         let Some(current) = surface.scopes.get(scope.0 as usize) else {
             return Ok(());
         };
-        if matches!(current.kind, ScopeKind::For { key_expr: None, .. }) {
+        if let ScopeKind::For {
+            pattern,
+            expr,
+            key_expr: None,
+        } = &current.kind
+        {
             return Err(io::Error::new(
                 io::ErrorKind::InvalidData,
                 format!(
-                    "{}: data-hemx-{kind}=\"{name}\" is inside an h-for without h-key; add a stable h-key=\"item.id\" to the loop that owns this generated target",
+                    "{}: data-hemx-{kind}=\"{name}\" is inside h-for=\"{pattern} in {expr}\" without h-key; add a stable h-key=\"{pattern}.id\" to that h-for so generated keyed helpers such as ui::{name}.replace(row) can target this partial. Dynamic +data-key on the child is rendered HTML, not the template fact hemx uses for generated targets.",
                     path.display()
                 ),
             ));
@@ -2052,22 +2057,25 @@ fn main() {{
 
     #[test]
     fn rejects_hemx_resources_inside_unkeyed_for() {
-        // req: scope/001
-        for (case, template, resource) in [
+        // req: scope/001 req: diagnostics/002
+        for (case, template, resource, helper) in [
             (
                 "slot",
                 r#"<template h-for="todo in &self.todos"><li data-hemx-slot="todo_row">{+ todo.title +}</li></template>"#,
                 "data-hemx-slot=\"todo_row\"",
+                "ui::todo_row",
             ),
             (
                 "slot_data_key",
                 r#"<template h-for="todo in &self.todos"><li data-hemx-slot="todo_row" +data-key="todo.id">{+ todo.title +}</li></template>"#,
                 "data-hemx-slot=\"todo_row\"",
+                "ui::todo_row",
             ),
             (
                 "handle",
                 r#"<template h-for="todo in &self.todos"><button data-hemx-handle="delete">Delete</button></template>"#,
                 "data-hemx-handle=\"delete\"",
+                "ui::delete",
             ),
         ] {
             let base = test_dir(&format!("hemx-build-unkeyed-for-{case}-test"));
@@ -2082,8 +2090,13 @@ fn main() {{
                 .out_dir(&out)
                 .run()
                 .unwrap_err();
-            assert!(err.to_string().contains("inside an h-for without h-key"));
-            assert!(err.to_string().contains(resource));
+            let err = err.to_string();
+            assert!(err.contains("inside h-for=\"todo in &self.todos\" without h-key"));
+            assert!(err.contains(resource));
+            assert!(err.contains("h-key=\"todo.id\""));
+            assert!(err.contains("generated keyed helpers"));
+            assert!(err.contains(helper));
+            assert!(err.contains("Dynamic +data-key on the child"));
 
             let _ = std::fs::remove_dir_all(&base);
         }