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/echo.rs

@@ -1181,7 +1181,7 @@ mod tests {
         // advanced` (verb + payload).
         for app in [
             AppCommand::Quit,
-            AppCommand::Help,
+            AppCommand::Help { topic: None },
             AppCommand::Rebuild,
             AppCommand::Save,
             AppCommand::New,