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/it/main.rs

@@ -11,6 +11,7 @@ mod case_insensitive_names;
 mod column_op_guards;
 mod engine_vocabulary_audit;
 mod friendly_enrichment;
+mod help_command;
 mod iteration2_persistence;
 mod iteration3_rebuild;
 mod iteration4a_rebuild_command;