hint: show the matching usage template for multi-form commands

A parse error in `add index …` showed the `add column` usage:
`add` and `drop` are multi-form commands, and both the
ambient hint and the submit-time usage block picked the
first-listed form unconditionally.

New `grammar::usage_key_for_input` disambiguates by the form
word after the entry keyword — `column` / `index` / `table` /
`relationship`, or the leading digit of `add 1:n …`. The
ambient hint now shows that one form; `render_usage_block`
shows the committed form's usage and falls back to the whole
family only for a bare `add` / `drop` with no form chosen.

src/input_render.rs

@@ -341,8 +341,10 @@ pub fn ambient_hint(
                 }
             } else {
                 let _ = position;
-                let usage = crate::dsl::grammar::usage_keys_for_input(input)
-                    .and_then(|(_, keys)| keys.first().copied())
+                // The form the user has committed to drives the
+                // usage template — `add index …` shows the
+                // `add index` usage, not the first `add` form.
+                let usage = crate::dsl::grammar::usage_key_for_input(input)
                     .map(|key| crate::friendly::translate(key, &[]));
                 Some(AmbientHint::Prose(match usage {
                     Some(u) => crate::t!(
@@ -876,6 +878,25 @@ mod tests {
         assert_eq!(cs, vec!["data".to_string(), "table".to_string()]);
     }
 
+    #[test]
+    fn ambient_hint_usage_matches_the_add_form_typed() {
+        // A trailing-junk error after `add index …` must show
+        // the `add index` usage — `add` is a multi-form
+        // command and the hint used to always show the first
+        // form (`add column`).
+        let input = "add index on Customers (barg):";
+        let h = prose(input, input.len()).expect("prose hint");
+        assert!(h.contains("usage:"), "got {h:?}");
+        assert!(
+            h.contains("add index"),
+            "should show the `add index` usage, got {h:?}",
+        );
+        assert!(
+            !h.contains("add column"),
+            "should not show the `add column` usage, got {h:?}",
+        );
+    }
+
     #[test]
     fn ambient_hint_for_definite_error_includes_usage_template() {
         let h = prose("insert into T extra", 19).expect("prose hint");