fix(build): validate slhx convention placement

Reject data-slhx-nav on non-anchor elements and data-slhx-sse away from its root, so standard conventions keep progressive navigation and root-scoped push semantics instead of silently doing nothing.

req: page_swap/001

req: push/006

req: diagnostics/002

REQUIREMENTS.md

@@ -485,7 +485,7 @@ what a valid business email is.
 005	Push is one-way server-to-client delivery of EffectBatch. It does not define client mutation, optimistic queues, reconciliation, or conflict handling.
 
 ### req: push/006
-006	`data-slhx-sse` opens only non-empty same-origin SSE URLs by default. Empty static URLs are build errors; cross-origin streams belong to explicit integration code rather than the standard runtime convention.
+006	`data-slhx-sse` is declared on `data-slhx-root` and opens only non-empty same-origin SSE URLs by default. Empty static URLs and non-root placement are build errors; cross-origin streams belong to explicit integration code rather than the standard runtime convention.
 
 ---
 

slhx-build/src/lib.rs

@@ -152,6 +152,7 @@ impl Resources {
             reject_selector_target_attrs(path, &node.attrs)?;
             reject_unknown_slhx_attrs(path, &node.attrs)?;
             reject_invalid_slhx_attr_values(path, &node.attrs)?;
+            reject_invalid_slhx_attr_placement(path, tag, &node.attrs)?;
 
             if let Some(class_attr) = static_attr(&node.attrs, "class") {
                 for token in class_tokens(&class_attr) {
@@ -1065,6 +1066,38 @@ fn reject_invalid_slhx_attr_values(path: &Path, attrs: &[SurfaceAttribute]) -> i
     Ok(())
 }
 
+fn reject_invalid_slhx_attr_placement(path: &Path, tag: &str, attrs: &[SurfaceAttribute]) -> io::Result<()> {
+    if has_attr(attrs, "data-slhx-nav") && tag != "a" {
+        return Err(invalid_slhx_placement(
+            path,
+            "data-slhx-nav",
+            "expected a real `<a href=...>` link so navigation works without JavaScript",
+        ));
+    }
+    if has_attr(attrs, "data-slhx-sse") && !has_attr(attrs, "data-slhx-root") {
+        return Err(invalid_slhx_placement(
+            path,
+            "data-slhx-sse",
+            "expected placement on the same element as `data-slhx-root`",
+        ));
+    }
+    Ok(())
+}
+
+fn has_attr(attrs: &[SurfaceAttribute], name: &str) -> bool {
+    attrs.iter().any(|attr| attr.name == name)
+}
+
+fn invalid_slhx_placement(path: &Path, attr: &str, expectation: &str) -> io::Error {
+    io::Error::new(
+        io::ErrorKind::InvalidData,
+        format!(
+            "{}: invalid {attr} placement; {expectation}",
+            path.display()
+        ),
+    )
+}
+
 fn valid_policy(value: &str) -> bool {
     matches!(value.trim(), "latest" | "queue" | "drop" | "parallel")
 }
@@ -1609,6 +1642,39 @@ fn main() {{
         }
     }
 
+    #[test]
+    fn rejects_invalid_slhx_attr_placement() {
+        // req: page_swap/001 req: push/006 req: diagnostics/002
+        for (case, template, attr, guidance) in [
+            (
+                "nav-button",
+                r#"<button data-slhx-nav="" data-slhx-handle="go">Go</button>"#,
+                "data-slhx-nav",
+                "<a href=...>",
+            ),
+            (
+                "sse-child",
+                r#"<section data-slhx-root="feed"><div data-slhx-sse="/events"></div></section>"#,
+                "data-slhx-sse",
+                "data-slhx-root",
+            ),
+        ] {
+            let base = test_dir(&format!("slhx-build-invalid-placement-{case}-test"));
+            let templates = base.join("templates");
+            let out = base.join("out");
+            let _ = std::fs::remove_dir_all(&base);
+            std::fs::create_dir_all(&templates).unwrap();
+            std::fs::write(templates.join("placement.heml"), template).unwrap();
+
+            let err = app().template_dir(&templates).out_dir(&out).run().unwrap_err();
+            assert!(err.to_string().contains(attr), "missing attr in diagnostic: {err}");
+            assert!(err.to_string().contains("invalid"), "missing invalid-placement diagnostic: {err}");
+            assert!(err.to_string().contains(guidance), "missing placement guidance {guidance:?}: {err}");
+
+            let _ = std::fs::remove_dir_all(&base);
+        }
+    }
+
     #[test]
     fn generated_lowering_injects_progressive_form_handle_and_key() {
         // req: form/002, req: wire/001