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].

tests/typing_surface/mod.rs

@@ -246,7 +246,7 @@ fn command_kind_label(cmd: &rdbms_playground::dsl::Command) -> String {
         SqlDelete { .. } => "SqlDelete".into(),
         App(app) => match app {
             AppCommand::Quit => "App(Quit)".into(),
-            AppCommand::Help => "App(Help)".into(),
+            AppCommand::Help { .. } => "App(Help)".into(),
             AppCommand::Rebuild => "App(Rebuild)".into(),
             AppCommand::Save => "App(Save)".into(),
             AppCommand::SaveAs => "App(SaveAs)".into(),