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/parse_error_pedagogy.rs

@@ -72,7 +72,10 @@ fn near_miss_matrix_simple_mode() {
         // trailing junk with "expected end of input" + their usage
         // (audited 2026-06-05); locked here as regression insurance.
         ("quit now", &["after `quit`, expected end of input", "  quit"]),
-        ("help foo", &["after `help`, expected end of input", "  help"]),
+        // `help` now takes an optional single-word topic (H3), so
+        // `help foo` parses (topic lookup); only a *multi-word*
+        // topic is the near-miss that rejects trailing junk.
+        ("help foo bar", &["after `help foo`, expected end of input", "help [<command>]"]),
         ("rebuild now", &["after `rebuild`, expected end of input", "  rebuild"]),
         ("new foo", &["after `new`, expected end of input", "  new"]),
         ("load foo", &["after `load`, expected end of input", "  load"]),