feat: H3 help <command> per-command detail + general reference

HELP node takes an optional single-word topic (BarePath);
AppCommand::Help { topic }. note_help_topic renders the help
block(s) of every command sharing that entry word (so `help
create` covers both create forms), plus `help types` and a
friendly "no help for X" pointer for unknown topics. Full help
gains a detail-hint footer. Catalogued help.detail_hint /
help.unknown_topic; parse-error matrix updated (help now takes a
topic, so the near-miss is the multi-word case). 9 integration
tests in tests/it/help_command.rs. Mark H3 [x].

src/app.rs

@@ -1321,8 +1321,11 @@ impl App {
         use crate::dsl::{AppCommand, MessagesValue, ModeValue};
         match cmd {
             AppCommand::Quit => vec![Action::Quit],
-            AppCommand::Help => {
-                self.note_help();
+            AppCommand::Help { topic } => {
+                match &topic {
+                    Some(t) => self.note_help_topic(t),
+                    None => self.note_help(),
+                }
                 Vec::new()
             }
             AppCommand::Rebuild => vec![Action::PrepareRebuild],
@@ -2405,6 +2408,49 @@ impl App {
                 .lines()
                 .map(str::to_string),
         );
+        // H3: point at the focused per-command form.
+        lines.push(crate::t!("help.detail_hint"));
+        for line in lines {
+            self.note_system(line);
+        }
+    }
+
+    /// Focused per-command help (H3): `help <topic>`, where `topic`
+    /// is a command entry word (`insert`, `create`, `show`, …) or
+    /// the special `types`. Renders the help block(s) of every
+    /// command sharing that entry word — so `help create` covers
+    /// both the DSL and SQL create forms — or a friendly pointer
+    /// back to `help` when nothing matches.
+    fn note_help_topic(&mut self, topic: &str) {
+        use crate::dsl::grammar::REGISTRY;
+
+        let topic = topic.trim();
+        // `help types` re-shows just the type reference.
+        if topic.eq_ignore_ascii_case("types") {
+            for line in crate::t!("help.types_reference").lines() {
+                self.note_system(line.to_string());
+            }
+            return;
+        }
+
+        let mut lines: Vec<String> = Vec::new();
+        for (command, _category) in REGISTRY {
+            let Some(help_id) = command.help_id else {
+                continue;
+            };
+            if command.entry.matches(topic) {
+                let key = format!("help.{help_id}");
+                let body = crate::friendly::translate(&key, &[]);
+                lines.extend(body.lines().map(str::to_string));
+            }
+        }
+
+        if lines.is_empty() {
+            // No command owns that entry word — name it and point
+            // back at the full list rather than failing silently.
+            self.note_system(crate::t!("help.unknown_topic", topic = topic));
+            return;
+        }
         for line in lines {
             self.note_system(line);
         }