fix(build): reject empty slhx guards

Treat empty static data-slhx-on lists and empty data-slhx-confirm messages as build errors, so authors do not silently disable dispatch or confirmation guards with blank attributes.

req: convention/002

req: convention/004

req: diagnostics/002

REQUIREMENTS.md

@@ -870,7 +870,7 @@ All forms support returning `impl IntoEffect` and compose through tuples.
 003	`data-slhx-debounce` and `data-slhx-throttle` support simple millisecond values. No trigger mini-language in core.
 
 ### req: convention/004
-004	`data-slhx-confirm` dispatches a native `confirm()` before handler dispatch. Custom confirm UI belongs to integration crates.
+004	`data-slhx-confirm` dispatches a native `confirm()` before handler dispatch. Static empty confirmation messages are build errors because they silently disable the guard in browsers. Custom confirm UI belongs to integration crates.
 
 ### req: convention/005
 005	`data-slhx-every` dispatches a handle at a fixed interval while the element remains in the document. Duplicate timers per root are avoided.

slhx-build/src/lib.rs

@@ -1015,9 +1015,7 @@ fn known_slhx_attr(name: &str) -> bool {
 
 fn reject_invalid_slhx_attr_values(path: &Path, attrs: &[SurfaceAttribute]) -> io::Result<()> {
     for attr in attrs.iter().filter(|attr| attr.origin == AttributeOrigin::Static) {
-        let Some(value) = attr.value.as_deref() else {
-            continue;
-        };
+        let value = attr.value.as_deref().unwrap_or("");
         match attr.name.as_str() {
             "data-slhx-policy" if !valid_policy(value) => {
                 return Err(invalid_slhx_value(
@@ -1027,7 +1025,7 @@ fn reject_invalid_slhx_attr_values(path: &Path, attrs: &[SurfaceAttribute]) -> i
                     "expected one of `latest`, `queue`, `drop`, or `parallel`",
                 ));
             }
-            "data-slhx-on" if !event_tokens(value).all(valid_runtime_event) => {
+            "data-slhx-on" if !valid_event_list(value) => {
                 return Err(invalid_slhx_value(
                     path,
                     &attr.name,
@@ -1035,6 +1033,14 @@ fn reject_invalid_slhx_attr_values(path: &Path, attrs: &[SurfaceAttribute]) -> i
                     "expected runtime-supported events: `click`, `submit`, `input`, `change`, `dragstart`, `dragover`, or `drop`",
                 ));
             }
+            "data-slhx-confirm" if value.trim().is_empty() => {
+                return Err(invalid_slhx_value(
+                    path,
+                    &attr.name,
+                    value,
+                    "expected a non-empty confirmation message",
+                ));
+            }
             "data-slhx-debounce" | "data-slhx-throttle" | "data-slhx-every"
                 if !valid_duration(value) =>
             {
@@ -1055,6 +1061,11 @@ fn valid_policy(value: &str) -> bool {
     matches!(value.trim(), "latest" | "queue" | "drop" | "parallel")
 }
 
+fn valid_event_list(value: &str) -> bool {
+    let mut events = event_tokens(value).peekable();
+    events.peek().is_some() && events.all(valid_runtime_event)
+}
+
 fn valid_runtime_event(value: &str) -> bool {
     matches!(value, "click" | "submit" | "input" | "change" | "dragstart" | "dragover" | "drop")
 }
@@ -1529,7 +1540,7 @@ fn main() {{
 
     #[test]
     fn rejects_invalid_static_slhx_convention_values() {
-        // req: convention/003 req: convention/005 req: convention/006 req: diagnostics/002
+        // req: convention/002 req: convention/003 req: convention/004 req: convention/005 req: convention/006 req: diagnostics/002
         for (case, template, attr, guidance) in [
             (
                 "policy",
@@ -1555,6 +1566,18 @@ fn main() {{
                 "data-slhx-on",
                 "click",
             ),
+            (
+                "empty-event",
+                r#"<button data-slhx-handle="save" data-slhx-on="">Save</button>"#,
+                "data-slhx-on",
+                "click",
+            ),
+            (
+                "empty-confirm",
+                r#"<button data-slhx-handle="delete" data-slhx-confirm="">Delete</button>"#,
+                "data-slhx-confirm",
+                "non-empty",
+            ),
         ] {
             let base = test_dir(&format!("slhx-build-invalid-convention-{case}-test"));
             let templates = base.join("templates");