docs: document custom type support throughout project

Update all documentation to reflect that custom types work automatically
with the #[task] macro, requiring only Clone + Send + Sync + 'static.

Documentation updates:
- README: Add custom types to features, new Custom Types section
- CHANGELOG: Document custom type improvement in [Unreleased]
- src/lib.rs: Add custom types example to main API docs
- src/task.rs: Update Task trait docs to mention macro-generated extraction
- src/extract.rs: Clarify ExtractInput is now internal-only (for task_fn)
- dagx-macros: Update macro docs to emphasize automatic type handling

New example:
- examples/custom_types.rs: Simple 120-line example showing User type
  flowing through DAG with fan-out pattern (one output → two consumers)
  demonstrating Arc wrapping is automatic and transparent

This makes the custom type capability discoverable and prominent for users,
turning a previous limitation into a headline feature.

dagx-macros/src/lib.rs

@@ -12,10 +12,15 @@ use syn::{parse_macro_input, FnArg, ImplItem, ItemImpl, Pat, PatType, ReturnType
 /// Apply this to an `impl` block containing a `run()` method (sync or async). The macro:
 /// - Derives `Input` and `Output` types from the `run()` signature
 /// - Automatically implements the `Task` trait
+/// - **Generates type-specific extraction logic** - works with ANY type (Clone + Send + Sync)!
 /// - Supports both sync and async run methods
 /// - Supports stateless (no self) and stateful (&self, &mut self) tasks
 /// - Handles various input patterns (no inputs, single input, multiple inputs)
 ///
+/// **Key Feature**: Custom types work automatically without implementing any traits!
+/// The macro generates inline extraction logic in `extract_and_run()` specific to your
+/// task's parameter types. Just derive `Clone` on your types and they'll work seamlessly.
+///
 /// # Task Patterns
 ///
 /// The `#[task]` macro supports three patterns based on state requirements: