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`.

2 files changed, 72 insertions, 13 deletions

diff --git a/mingling/src/example_docs.rs b/mingling/src/example_docs.rs
index ea8e539..266a449 100644
--- a/mingling/src/example_docs.rs
+++ b/mingling/src/example_docs.rs
@@ -1212,7 +1212,7 @@ pub mod example_exitcode {}
 /// Source code (./src/main.rs)
 /// ```ignore
 /// 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);
@@ -1228,11 +1228,12 @@ pub mod example_exitcode {}
 /// // --------- 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,
@@ -1686,21 +1687,23 @@ pub mod example_outside_type {}
 ///  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"}
 ///  ```
 ///
 /// Source code (./Cargo.toml)
@@ -1730,6 +1733,7 @@ pub mod example_outside_type {}
 /// use std::path::PathBuf;
 ///
 /// dispatcher!("find", CMDFind => EntryFind);
+/// dispatcher!("find-structural", CMDFindStructural => EntryFindStructural);
 ///
 /// // --------- IMPORTANT ---------
 /// // `pack_err!` is a convenient macro for defining error types.
@@ -1753,8 +1757,14 @@ pub mod example_outside_type {}
 /// // 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 {
@@ -1773,6 +1783,23 @@ pub mod example_outside_type {}
 ///     }
 /// }
 ///
+/// #[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) {
@@ -1791,6 +1818,18 @@ pub mod example_outside_type {}
 ///     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() {
@@ -1798,6 +1837,7 @@ pub mod example_outside_type {}
 ///     // 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/mingling/src/lib.rs b/mingling/src/lib.rs
index 70b69bc..4c49f15 100644
--- a/mingling/src/lib.rs
+++ b/mingling/src/lib.rs
@@ -96,15 +96,24 @@ pub mod macros {
     /// Used to register an external type as a group member
     #[cfg(feature = "extra_macros")]
     pub use mingling_macros::group;
+    /// Like `group!` but also marks the type for structured output
+    #[cfg(all(feature = "general_renderer", feature = "extra_macros"))]
+    pub use mingling_macros::group_structural;
     /// Used to generate a struct implementing the `HelpRequest` trait via a method
     pub use mingling_macros::help;
     /// Used to create a `Node` struct via a literal
     pub use mingling_macros::node;
     /// Used to create a wrapper type for use with `Chain` and `Renderer`
     pub use mingling_macros::pack;
+    /// Like `pack!` but also marks the type for structured output
+    #[cfg(feature = "general_renderer")]
+    pub use mingling_macros::pack_structural;
     /// Used to create an error struct with automatic `name` field
     #[cfg(feature = "extra_macros")]
     pub use mingling_macros::pack_err;
+    /// Like `pack_err!` but also marks the type for structured output
+    #[cfg(all(feature = "general_renderer", feature = "extra_macros"))]
+    pub use mingling_macros::pack_err_structural;
     #[cfg(feature = "comp")]
     #[doc(hidden)]
     pub use mingling_macros::program_comp_gen;
@@ -148,6 +157,10 @@ pub use mingling_macros::EnumTag;
 /// derive macro Groupped
 pub use mingling_macros::Groupped;
 
+/// derive macro `StructuralData` — marks a type as supporting structured output
+#[cfg(feature = "general_renderer")]
+pub use mingling_macros::StructuralData;
+
 /// Example projects for `Mingling`, for learning how to use `Mingling`
 pub mod _mingling_examples {
     pub use crate::example_docs::*;
@@ -200,9 +213,15 @@ pub mod prelude {
     pub use crate::macros::gen_program;
     /// Re-export of the `pack` macro for creating wrapper types.
     pub use crate::macros::pack;
+    /// Like `pack!` but also marks the type for structured output
+    #[cfg(feature = "general_renderer")]
+    pub use mingling_macros::pack_structural;
     /// Re-export of the `pack_err` macro for creating error types.
     #[cfg(feature = "extra_macros")]
     pub use crate::macros::pack_err;
+    /// Like `pack_err!` but also marks the type for structured output
+    #[cfg(all(feature = "general_renderer", feature = "extra_macros"))]
+    pub use mingling_macros::pack_err_structural;
     /// Re-export of the `r_print` macro for printing within a renderer context.
     pub use crate::macros::r_print;
     /// Re-export of the `r_println` macro for printing with a newline within a renderer