fix(build): require href on slhx navigation

Treat static data-slhx-nav anchors without href as build errors so page-swap links preserve progressive navigation when JavaScript is unavailable.

req: page_swap/001

req: diagnostics/002

REQUIREMENTS.md

@@ -121,7 +121,7 @@ slhx competes with React by making frontend frameworks unnecessary for most apps
 ## page_swap
 
 ### req: page_swap/001
-001	Minimal page swapping is a first-class slhx-axum happy path. Authors mark real anchors with `data-slhx-nav`; links keep valid `href` and work without JS.
+001	Minimal page swapping is a first-class slhx-axum happy path. Authors mark real anchors with `data-slhx-nav`; links keep valid `href` and work without JS. Missing `href` on a static `data-slhx-nav` anchor is a build error.
 
 ### req: page_swap/002
 002	A `data-slhx-nav` click fetches the target URL as a slhx partial request. The response updates the canonical content slot, optionally navigation and title, then applies a `Navigate` effect.

slhx-build/src/lib.rs

@@ -1067,7 +1067,7 @@ fn reject_invalid_slhx_attr_values(path: &Path, attrs: &[SurfaceAttribute]) -> i
 }
 
 fn reject_invalid_slhx_attr_placement(path: &Path, tag: &str, attrs: &[SurfaceAttribute]) -> io::Result<()> {
-    if has_attr(attrs, "data-slhx-nav") && tag != "a" {
+    if has_attr(attrs, "data-slhx-nav") && (tag != "a" || !has_attr(attrs, "href")) {
         return Err(invalid_slhx_placement(
             path,
             "data-slhx-nav",
@@ -1652,6 +1652,12 @@ fn main() {{
                 "data-slhx-nav",
                 "<a href=...>",
             ),
+            (
+                "nav-missing-href",
+                r#"<a data-slhx-nav="">Docs</a>"#,
+                "data-slhx-nav",
+                "<a href=...>",
+            ),
             (
                 "sse-child",
                 r#"<section data-slhx-root="feed"><div data-slhx-sse="/events"></div></section>"#,