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/dsl/command.rs

@@ -492,8 +492,14 @@ pub enum Command {
 pub enum AppCommand {
     /// Exit cleanly. Accepts the `q` alias.
     Quit,
-    /// Show in-app help. Body comes from `help.in_app_body`.
-    Help,
+    /// Show in-app help (H3). With no `topic`, the full command
+    /// list + types reference; with a `topic` (a command entry
+    /// word like `insert` / `create` / `show`, or `types`), the
+    /// focused detail for that command (or command group sharing
+    /// the entry word).
+    Help {
+        topic: Option<String>,
+    },
     /// Rebuild `playground.db` from `project.yaml` + data/, with
     /// confirmation modal.
     Rebuild,
@@ -910,7 +916,7 @@ impl Command {
             Self::SqlDelete { .. } => "delete from",
             Self::App(app) => match app {
                 AppCommand::Quit => "quit",
-                AppCommand::Help => "help",
+                AppCommand::Help { .. } => "help",
                 AppCommand::Rebuild => "rebuild",
                 AppCommand::Save => "save",
                 AppCommand::SaveAs => "save as",