fix(build): reject boost on action elements

Treat data-slhx-boost on anchors and forms as a build error with guidance toward data-slhx-nav or data-slhx-handle, preserving boost as a container convention.

req: page_swap/007

req: diagnostics/002

slhx-build/src/lib.rs

@@ -1074,6 +1074,13 @@ fn reject_invalid_slhx_attr_placement(path: &Path, tag: &str, attrs: &[SurfaceAt
             "expected a real `<a href=...>` link so navigation works without JavaScript",
         ));
     }
+    if has_attr(attrs, "data-slhx-boost") && matches!(tag, "a" | "form") {
+        return Err(invalid_slhx_placement(
+            path,
+            "data-slhx-boost",
+            "expected a container around descendant links/forms; use `data-slhx-nav` on anchors or `data-slhx-handle` on forms",
+        ));
+    }
     if has_attr(attrs, "data-slhx-sse") && !has_attr(attrs, "data-slhx-root") {
         return Err(invalid_slhx_placement(
             path,
@@ -1644,7 +1651,7 @@ fn main() {{
 
     #[test]
     fn rejects_invalid_slhx_attr_placement() {
-        // req: page_swap/001 req: push/006 req: diagnostics/002
+        // req: page_swap/001 req: page_swap/007 req: push/006 req: diagnostics/002
         for (case, template, attr, guidance) in [
             (
                 "nav-button",
@@ -1658,6 +1665,18 @@ fn main() {{
                 "data-slhx-nav",
                 "<a href=...>",
             ),
+            (
+                "boost-anchor",
+                r#"<a href="/docs" data-slhx-boost="">Docs</a>"#,
+                "data-slhx-boost",
+                "data-slhx-nav",
+            ),
+            (
+                "boost-form",
+                r#"<form data-slhx-boost=""><input name="q"></form>"#,
+                "data-slhx-boost",
+                "data-slhx-handle",
+            ),
             (
                 "sse-child",
                 r#"<section data-slhx-root="feed"><div data-slhx-sse="/events"></div></section>"#,