Send --help to stdout; reject duplicate subcommand names

Sixth codex pass:

- help: split HelpPrinter into print() (stdout, for user-requested
  help) and print_error() (stderr, grouped with the error message).
  Piping `cmd --help > file` now works.

- dispatch: LauncherWithSubs::command() panics on a duplicate name. A
  second registration was silently overshadowed because parsing stops
  at the first match, so the later handler was dead code. Panicking
  at startup makes the programmer error impossible to miss.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: xerial

runi-cli/src/launcher/dispatch.rs

@@ -104,6 +104,13 @@ impl<G: Command + 'static> LauncherWithSubs<G> {
     where
         S: Command + SubCommandOf<G> + 'static,
     {
+        // Silently accepting a duplicate would make later registrations
+        // unreachable because parsing stops at the first match. That's a
+        // programmer error — fail loudly at startup.
+        assert!(
+            !self.subs.iter().any(|e| e.schema.name == name),
+            "duplicate subcommand name: {name}",
+        );
         let mut schema = S::schema();
         schema.name = name.to_string();
         let name_owned = schema.name.clone();
@@ -192,14 +199,14 @@ fn report_error(err: Error, root: &CommandSchema) -> i32 {
                 }
                 inner => {
                     eprintln!("error: {inner}");
-                    HelpPrinter::print(schema);
+                    HelpPrinter::print_error(schema);
                     2
                 }
             }
         }
         other => {
             eprintln!("error: {other}");
-            HelpPrinter::print(root);
+            HelpPrinter::print_error(root);
             2
         }
     }
@@ -486,6 +493,14 @@ mod tests {
         }
     }
 
+    #[test]
+    #[should_panic(expected = "duplicate subcommand name: clone")]
+    fn duplicate_subcommand_registration_panics() {
+        let _ = Launcher::<GitApp>::of()
+            .command::<CloneCmd>("clone")
+            .command::<CloneCmd>("clone");
+    }
+
     #[test]
     fn compose_help_schema_prefixes_root_name_and_options() {
         let root = CommandSchema::new("git", "").flag("-v,--verbose", "Verbose");

runi-cli/src/launcher/help.rs

@@ -52,8 +52,15 @@ impl HelpPrinter {
         out
     }
 
-    /// Print help text to stderr.
+    /// Print help text to stdout. Use this for user-requested help
+    /// (`--help`) so output can be piped or redirected normally.
     pub fn print(schema: &CommandSchema) {
+        print!("{}", Self::format(schema));
+    }
+
+    /// Print help text to stderr. Use this alongside an error message so
+    /// both are grouped on the same stream.
+    pub fn print_error(schema: &CommandSchema) {
         eprint!("{}", Self::format(schema));
     }
 }