fix(build): reject empty slhx navigation hrefs

Treat data-slhx-nav anchors with empty static href values as build errors, preserving progressive navigation instead of letting links depend on JavaScript-only behavior.

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. Missing `href` on a static `data-slhx-nav` anchor is a build error.
+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 or empty static `href` on a `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 +1067,9 @@ 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" || !has_attr(attrs, "href")) {
+    if has_attr(attrs, "data-slhx-nav")
+        && (tag != "a" || !has_attr(attrs, "href") || static_attr(attrs, "href").is_some_and(|href| href.trim().is_empty()))
+    {
         return Err(invalid_slhx_placement(
             path,
             "data-slhx-nav",
@@ -1665,6 +1667,12 @@ fn main() {{
                 "data-slhx-nav",
                 "<a href=...>",
             ),
+            (
+                "nav-empty-href",
+                r#"<a href="" data-slhx-nav="">Docs</a>"#,
+                "data-slhx-nav",
+                "<a href=...>",
+            ),
             (
                 "boost-anchor",
                 r#"<a href="/docs" data-slhx-boost="">Docs</a>"#,