Parser: parse `trait` and `impl` bodies into the AST

[StreamDemon]

Summary

trait_def and impl_block skip their bodies (skip_item_body) — associated items (method signatures, associated types/consts, default methods, impl methods) are not parsed into the AST. Trait stores only a name; ImplBlock stores only a target type.

Part of the v0.6.x parser-completeness work (see crates/AGENTS.md "Implemented subset").

Spec

• §3.5 traits/impls, supertraits; §16 trait_item / impl_item. (The §10 orphan rule is a later semantic concern, not parsing.)

Scope

• Parse trait bodies: method signatures (no body), default methods (with body), associated type / const.
• Parse impl bodies: methods and associated items; capture impl Trait for Type vs. inherent impl Type, and any supertrait/for target.
• AST nodes for trait/impl items.
• Corpus + unit tests.

Notes

Reuses function_after_mods for methods. Coordinate with the generics issue so impl<T> / trait T<U> parameters are captured.