feat: add entry metadata system with compile-time typed values

Add `Metadata<B>` trait and `#[metadata(Entry)]` attribute macro
to attach arbitrary, compile-time-typed metadata to entries.
Implement `ProgramCollect::get_metadata<T>()` for runtime retrieval,
backed by a global registry populated by `register_metadata!`.

Extend `pathf` with `MetadataPattern` to resolve metadata types
across modules at build time. Add two examples demonstrating
metadata usage with and without pathf integration.

4 files changed, 28 insertions, 0 deletions

diff --git a/mingling_core/src/asset.rs b/mingling_core/src/asset.rs
index 8c709ac..fc1c81b 100644
--- a/mingling_core/src/asset.rs
+++ b/mingling_core/src/asset.rs
@@ -5,6 +5,7 @@ pub(crate) mod enum_tag;
 pub(crate) mod global_resource;
 pub(crate) mod help;
 pub(crate) mod lazy_resource;
+pub(crate) mod metadata;
 pub(crate) mod node;
 pub(crate) mod renderer;
 pub(crate) mod routable;
diff --git a/mingling_core/src/asset/metadata.rs b/mingling_core/src/asset/metadata.rs
new file mode 100644
index 0000000..996b34c
--- /dev/null
+++ b/mingling_core/src/asset/metadata.rs
@@ -0,0 +1,14 @@
+/// Provides metadata for an Entry.
+///
+/// Any type can be attached to an Entry as metadata, allowing the program to
+/// carry compile-time-typed, arbitrary description data alongside each
+/// registered entry. The [`Metadata`] trait bridges an Entry type (`Self`) to
+/// an arbitrary metadata type `B`.
+///
+/// It is recommended to use the `#[metadata(Entry)]` attribute macro from
+/// [mingling_macros](https://crates.io/crates/mingling_macros) to implement this
+/// trait and register the entry via `register_metadata!`.
+pub trait Metadata<B> {
+    /// Initializes and returns the metadata value of type `B` for this entry.
+    fn init_metadata() -> B;
+}
diff --git a/mingling_core/src/lib.rs b/mingling_core/src/lib.rs
index cd42842..3a5acf4 100644
--- a/mingling_core/src/lib.rs
+++ b/mingling_core/src/lib.rs
@@ -70,6 +70,7 @@ pub use crate::asset::enum_tag::*;
 pub use crate::asset::global_resource::*;
 pub use crate::asset::help::*;
 pub use crate::asset::lazy_resource::*;
+pub use crate::asset::metadata::*;
 pub use crate::asset::node::*;
 pub use crate::asset::renderer::*;
 pub use crate::asset::routable::*;
diff --git a/mingling_core/src/program/collection.rs b/mingling_core/src/program/collection.rs
index 1b4d7dd..5b1152a 100644
--- a/mingling_core/src/program/collection.rs
+++ b/mingling_core/src/program/collection.rs
@@ -66,6 +66,18 @@ pub trait ProgramCollect {
     /// Render help for Entry
     fn render_help(any: AnyOutput<Self::Enum>) -> RenderResult;
 
+    /// Retrieves compile-time registered metadata of type `T` for the given
+    /// enum member, if any was registered via `#[metadata(Entry)]`.
+    ///
+    /// Returns `None` when no metadata of type `T` has been registered for the
+    /// provided member, or when the requested `T` does not match the registered
+    /// metadata type. The concrete implementation of this method is generated
+    /// by the `gen_program!` macro.
+    fn get_metadata<T: 'static>(member_id: Self::Enum) -> Option<T> {
+        let _ = member_id;
+        None
+    }
+
     /// Find a matching chain to continue execution based on the input [AnyOutput](./struct.AnyOutput.html), returning a new [AnyOutput](./struct.AnyOutput.html)
     #[cfg(feature = "async")]
     fn do_chain(