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/dsl/grammar/app.rs

@@ -80,6 +80,11 @@ const IMPORT_PATH_AND_TARGET: Node = Node::Seq(&[Node::BarePath, IMPORT_AS_TARGE
 const EXPORT_PATH_OPT: Node = Node::Optional(&Node::BarePath);
 const IMPORT_BODY_OPT: Node = Node::Optional(&IMPORT_PATH_AND_TARGET);
 
+// `help [<topic>]` (H3): an optional single-word topic — a command
+// entry word (`insert`, `create`, `show`, …) or `types`. Captured
+// as a `BarePath` so any keyword-shaped word is accepted verbatim.
+const HELP_TOPIC_OPT: Node = Node::Optional(&Node::BarePath);
+
 // `mode <value>`: known keywords are surfaced as `Word` children
 // so they appear in the walker's expected set (and feed the
 // completion engine's keyword candidates). The trailing `Ident`
@@ -154,8 +159,15 @@ const fn build_quit(_path: &MatchedPath, _source: &str) -> Result<Command, Valid
     Ok(Command::App(AppCommand::Quit))
 }
 
-const fn build_help(_path: &MatchedPath, _source: &str) -> Result<Command, ValidationError> {
-    Ok(Command::App(AppCommand::Help))
+fn build_help(path: &MatchedPath, _source: &str) -> Result<Command, ValidationError> {
+    // Optional single-word topic (a command entry word, or
+    // `types`) captured as a `BarePath` — `help insert`,
+    // `help create`, `help show`. Multi-word commands share an
+    // entry word, so `help create` covers every create form.
+    let topic = path
+        .find(|i| matches!(i.kind, MatchedKind::BarePath))
+        .map(|i| i.text.clone());
+    Ok(Command::App(AppCommand::Help { topic }))
 }
 
 const fn build_rebuild(_path: &MatchedPath, _source: &str) -> Result<Command, ValidationError> {
@@ -255,7 +267,7 @@ pub static QUIT: CommandNode = CommandNode {
 
 pub static HELP: CommandNode = CommandNode {
     entry: Word::keyword("help"),
-    shape: EMPTY_SEQ,
+    shape: HELP_TOPIC_OPT,
     ast_builder: build_help,
     help_id: Some("app.help"),
     usage_ids: &["parse.usage.help"],};