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/friendly/keys.rs

@@ -174,6 +174,8 @@ pub const KEYS_AND_PLACEHOLDERS: &[(&str, &[&str])] = &[
     ("help.intro", &[]),
     ("help.dsl_section", &[]),
     ("help.types_reference", &[]),
+    ("help.detail_hint", &[]),
+    ("help.unknown_topic", &["topic"]),
     ("help.app.quit", &[]),
     ("help.app.help", &[]),
     ("help.app.rebuild", &[]),