feat(core): decouple structured output from Groupped trait

Introduce `StructuralData` sealed trait and `pack_structural!` /
`group_structural!` / `derive(StructuralData)` macros to control
structured rendering separately from grouping. `Groupped` no longer
requires `Serialize`.

3 files changed, 69 insertions, 19 deletions

diff --git a/examples/example-general-renderer/src/main.rs b/examples/example-general-renderer/src/main.rs
index c923d28..1e02afb 100644
--- a/examples/example-general-renderer/src/main.rs
+++ b/examples/example-general-renderer/src/main.rs
@@ -18,7 +18,7 @@
 //! ```
 
 use mingling::prelude::*;
-use mingling::{parser::Picker, setup::GeneralRendererSetup, Groupped};
+use mingling::{parser::Picker, setup::GeneralRendererSetup, StructuralData, Groupped};
 use serde::Serialize;
 
 dispatcher!("render", CMDRender => EntryRender);
@@ -34,11 +34,12 @@ fn main() {
 // --------- IMPORTANT ---------
 // For beautiful output structure, do not use `pack!` to wrap the types that need to be output.
 // Instead, manually implement
-//        ____________________ Implement serde::Serialize
-//       /           _________ Implement mingling::Groupped
-//       |          /            to ensure Mingling can recognize the type
-//       vvvvvvvvv  vvvvvvvv
-#[derive(Serialize, Groupped)]
+//        __________________________________ Mark as structured data so it can be rendered
+//       /              ____________________ Implement serde::Serialize
+//       |             /           _________ Implement mingling::Groupped
+//       |             |          /            to ensure Mingling can recognize the type
+//       vvvvvvvvvvvv  vvvvvvvvv  vvvvvvvv
+#[derive(StructuralData, Serialize, Groupped)]
 struct Info {
     #[serde(rename = "member_name")]
     name: String,
diff --git a/examples/example-pack-err/src/main.rs b/examples/example-pack-err/src/main.rs
index 72fecd6..f859fae 100644
--- a/examples/example-pack-err/src/main.rs
+++ b/examples/example-pack-err/src/main.rs
@@ -7,21 +7,23 @@
 //! Run:
 //! ```bash
 //! cargo run --manifest-path examples/example-pack-err/Cargo.toml --quiet -- find
-//! cargo run --manifest-path examples/example-pack-err/Cargo.toml --quiet -- find --json
 //! cargo run --manifest-path examples/example-pack-err/Cargo.toml --quiet -- find Cargo.toml
-//! cargo run --manifest-path examples/example-pack-err/Cargo.toml --quiet -- find Cargo.toml --json
 //! cargo run --manifest-path examples/example-pack-err/Cargo.toml --quiet -- find src
-//! cargo run --manifest-path examples/example-pack-err/Cargo.toml --quiet -- find src --json
+//! cargo run --manifest-path examples/example-pack-err/Cargo.toml --quiet -- find-structural --json
+//! cargo run --manifest-path examples/example-pack-err/Cargo.toml --quiet -- find-structural Cargo.toml --json
+//! cargo run --manifest-path examples/example-pack-err/Cargo.toml --quiet -- find-structural src --json
 //! ```
 //!
 //! Output:
 //! ```plaintext
 //! Search path not provided
-//! {"name":"error_not_found"}
 //! Not a directory: Cargo.toml
-//! {"name":"error_not_dir","info":"Cargo.toml"}
 //! Found directory: src
+//! {"name":"error_not_found"}
+//! {"name":"error_not_dir","info":"Cargo.toml"}
 //! {"inner":"src"}
+//! {"name":"error_not_found_structural"}
+//! {"name":"error_not_dir_structural","info":"Cargo.toml"}
 //! ```
 
 use mingling::prelude::*;
@@ -29,6 +31,7 @@ use mingling::setup::GeneralRendererSetup;
 use std::path::PathBuf;
 
 dispatcher!("find", CMDFind => EntryFind);
+dispatcher!("find-structural", CMDFindStructural => EntryFindStructural);
 
 // --------- IMPORTANT ---------
 // `pack_err!` is a convenient macro for defining error types.
@@ -52,8 +55,14 @@ pack_err!(ErrorNotFound);
 // Typed form — name = "error_not_dir"
 pack_err!(ErrorNotDir = PathBuf);
 
-// Success type using traditional pack!
-pack!(ResultPath = PathBuf);
+// Simple form — with StructuralData support for --json / --yaml
+pack_err_structural!(ErrorNotFoundStructural);
+
+// Typed form — with StructuralData support for --json / --yaml
+pack_err_structural!(ErrorNotDirStructural = PathBuf);
+
+// Success type with StructuralData support
+pack_structural!(ResultPath = PathBuf);
 
 #[chain]
 fn handle_find(args: EntryFind) -> Next {
@@ -72,6 +81,23 @@ fn handle_find(args: EntryFind) -> Next {
     }
 }
 
+#[chain]
+fn handle_find_structural(args: EntryFindStructural) -> Next {
+    let Some(path_str) = args.inner.first().cloned() else {
+        // No path provided → use the simple error form (Default)
+        return ErrorNotFoundStructural::default().to_render();
+    };
+
+    let path = PathBuf::from(&path_str);
+    if path.is_dir() {
+        // Is a directory → success
+        ResultPath::new(path).to_render()
+    } else {
+        // Not a directory (or doesn't exist) → use the typed error form
+        ErrorNotDirStructural::new(path).to_render()
+    }
+}
+
 /// Renders the successful result with the found directory path.
 #[renderer]
 fn render_result_path(path: ResultPath) {
@@ -90,6 +116,18 @@ fn render_error_not_dir(err: ErrorNotDir) {
     r_println!("Not a directory: {}", err.info.display());
 }
 
+/// Renders the structural error when no search path is provided.
+#[renderer]
+fn render_error_not_found_structural(_: ErrorNotFoundStructural) {
+    r_println!("Search path not provided");
+}
+
+/// Renders the structural error when the given path is not a directory.
+#[renderer]
+fn render_error_not_dir_structural(err: ErrorNotDirStructural) {
+    r_println!("Not a directory: {}", err.info.display());
+}
+
 gen_program!();
 
 fn main() {
@@ -97,5 +135,6 @@ fn main() {
     // Add GeneralRendererSetup to support --json / --yaml flags
     program.with_setup(GeneralRendererSetup);
     program.with_dispatcher(CMDFind);
+    program.with_dispatcher(CMDFindStructural);
     let _ = program.exec();
 }
diff --git a/examples/test-examples.toml b/examples/test-examples.toml
index 9c25781..149f2c6 100644
--- a/examples/test-examples.toml
+++ b/examples/test-examples.toml
@@ -234,16 +234,26 @@ expect.exit-code = 0
 expect.result = "Search path not provided"
 
 [[test.example-pack-err]]
-command = "find --json"
+command = "find Cargo.toml"
 expect.exit-code = 0
-expect.result = '{"name":"error_not_found"}'
+expect.result = "Not a directory: Cargo.toml"
 
 [[test.example-pack-err]]
-command = "find Cargo.toml"
+command = "find examples"
 expect.exit-code = 0
-expect.result = "Not a directory: Cargo.toml"
+expect.result = "Found directory: examples"
+
+[[test.example-pack-err]]
+command = "find-structural --json"
+expect.exit-code = 0
+expect.result = '{"name":"error_not_found_structural"}'
+
+[[test.example-pack-err]]
+command = "find-structural Cargo.toml --json"
+expect.exit-code = 0
+expect.result = '{"name":"error_not_dir_structural","info":"Cargo.toml"}'
 
 [[test.example-pack-err]]
-command = "find Cargo.toml --json"
+command = "find-structural examples --json"
 expect.exit-code = 0
-expect.result = '{"name":"error_not_dir","info":"Cargo.toml"}'
+expect.result = '{"inner":"examples"}'