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.

1 files changed, 14 insertions, 0 deletions

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;
+}