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/dsl/grammar/mod.rs

@@ -414,6 +414,42 @@ pub fn usage_keys_for_input(source: &str) -> Option<(&'static str, &'static [&'s
     Some((node.entry.primary, node.usage_ids))
 }
 
+/// The single usage template most relevant to `source`, when
+/// one is determinable.
+///
+/// A single-form command resolves to its one usage key. A
+/// multi-form command (`add`, `drop`) disambiguates by the
+/// form word after the entry keyword — so a parse error in
+/// `add index …` resolves to the `add index` usage rather than
+/// the first-listed `add column`. Returns `None` for a bare
+/// multi-form entry word (`add` with nothing after it), where
+/// no form has been chosen — the caller decides whether to
+/// show the whole family or nothing.
+#[must_use]
+pub fn usage_key_for_input(source: &str) -> Option<&'static str> {
+    use crate::dsl::walker::lex_helpers::{consume_ident, skip_whitespace};
+    let (_entry, keys) = usage_keys_for_input(source)?;
+    let first = *keys.first()?;
+    if keys.len() == 1 {
+        return Some(first);
+    }
+    // Multi-form: the form is named by the token right after
+    // the entry keyword.
+    let start = skip_whitespace(source, 0);
+    let (_, entry_end) = consume_ident(source, start)?;
+    let after = skip_whitespace(source, entry_end);
+    // The `add 1:n relationship` form opens with a digit.
+    if source.as_bytes().get(after).is_some_and(u8::is_ascii_digit) {
+        return keys.iter().copied().find(|k| k.ends_with("relationship"));
+    }
+    // Otherwise the form word is an identifier — `column`,
+    // `index`, `table`, `relationship` — matched against the
+    // usage key's suffix.
+    let (s, e) = consume_ident(source, after)?;
+    let form = source[s..e].to_ascii_lowercase();
+    keys.iter().copied().find(|k| k.ends_with(form.as_str()))
+}
+
 /// Every command-entry word in the registry, sorted alphabetically
 /// by primary literal. Replaces `dsl::usage::entry_keywords_alphabetised`
 /// which read the same data through the legacy `usage::REGISTRY`.