feat(runtime): support viewport-ahead reveals

hemx-build/src/lib.rs

@@ -1623,6 +1623,7 @@ fn known_hemx_attr(name: &str) -> bool {
             | "data-hemx-every"
             | "data-hemx-interval"
             | "data-hemx-revealed"
+            | "data-hemx-revealed-ahead"
             | "data-hemx-disable-while-pending"
             | "data-hemx-policy"
             | "data-hemx-nav"
@@ -1727,6 +1728,20 @@ fn reject_invalid_hemx_attr_values(path: &Path, attrs: &[SurfaceAttribute]) -> i
                     "expected a non-empty same-origin SSE URL",
                 ));
             }
+            "data-hemx-revealed-ahead"
+                if !value
+                    .trim()
+                    .parse::<f64>()
+                    .ok()
+                    .is_some_and(|value| value.is_finite() && value >= 0.0) =>
+            {
+                return Err(invalid_hemx_value(
+                    path,
+                    &attr.name,
+                    value,
+                    "expected a non-negative number of viewports",
+                ));
+            }
             "data-hemx-debounce" | "data-hemx-delay" | "data-hemx-throttle" | "data-hemx-every"
             | "data-hemx-interval"
                 if !valid_duration(value) =>
@@ -2774,12 +2789,12 @@ mod tests {
         let canonical_syms = resources.syms();
         assert_eq!(
             stable_id("generated-rs", &canonical_generated),
-            3_748_714_336,
+            2_620_423_950,
             "canonical generated Rust changed"
         );
         assert_eq!(
             stable_id("generated-rs-global", &canonical_globals),
-            2_783_766_475,
+            2_559_847_213,
             "canonical global-export Rust changed"
         );
         assert_eq!(
@@ -4223,6 +4238,12 @@ fn main() {{
                 "data-hemx-delay",
                 "milliseconds",
             ),
+            (
+                "revealed-ahead-negative",
+                r#"<form data-hemx-revealed data-hemx-revealed-ahead="-1"></form>"#,
+                "data-hemx-revealed-ahead",
+                "non-negative",
+            ),
             (
                 "throttle-invalid",
                 r#"<button data-hemx-handle="save" data-hemx-throttle="ms">Save</button>"#,
@@ -4333,6 +4354,7 @@ fn main() {{
                 "data-hemx-client-state-version" => {
                     "expected a positive client state ABI version"
                 }
+                "data-hemx-revealed-ahead" => "expected a non-negative number of viewports",
                 "data-hemx-on" => "expected runtime-supported events: `click`, `submit`, `input`, `change`, `keydown`, `dragstart`, `dragover`, or `drop`",
                 "data-hemx-confirm" => "expected a non-empty confirmation message",
                 "data-hemx-sse" => "expected a non-empty same-origin SSE URL",