Add buffa-remote-derive: ProtoString/ProtoBytes/ProtoList/ProtoBox/MapStorage for remote types

[rsd-darshan]

Closes #212 — a #[derive(...)] #[buffa(remote = ...)] macro that generates the newtype boilerplate (Deref, AsRef/DerefMut, the relevant From impls, and the buffa trait itself) needed to use a foreign type as a custom owned field type.

Two tiers, one crate

ProtoString, ProtoBytes, ProtoList — the remote type's existing supertrait bounds (From<&str>, From<Vec<u8>>, FromIterator<T>, etc.) give the macro everything it needs; no extra configuration beyond naming the remote type.

#[derive(Clone, PartialEq, Default, Debug, buffa_remote_derive::ProtoString)]
#[buffa(remote = ecow::EcoString)]
pub struct MyEcoString(pub ecow::EcoString);

ProtoBox, MapStorage — their reference newtypes (examples/custom-types/src/types/small_box.rs, index_map.rs) call inherent methods on the remote type (smallbox::SmallBox::into_inner(), indexmap::IndexMap::insert()), not trait methods, so the macro can't synthesize the call purely from bounds. These default to the conventional method names (new/into_inner for pointers; len/insert/clear/iter for maps) with an attribute override for a remote type that names them differently:

#[derive(buffa_remote_derive::ProtoBox)]
#[buffa(remote = smallbox::SmallBox<T, smallbox::space::S4>)]
pub struct SmallBox<T>(pub smallbox::SmallBox<T, smallbox::space::S4>);

// A map whose insert method is named `put` instead:
#[derive(Clone, PartialEq, Debug, buffa_remote_derive::MapStorage)]
#[buffa(remote = ..., insert = MyMap::put)]
pub struct MyMapStorage<K, V>(pub MyMap<K, V>);

The override and the default are both rendered as UFCS-style calls (Type::method(&mut self.0, args...)), so a differently-named method with the same receiver shape drops in directly.

Notes

• The #[buffa(remote = ...)] attribute is documentation, not codegen input — the macro always reads the field's actual type. Comparing the attribute's value against the field's type isn't possible from within a derive macro without resolving use imports, so the attribute is required (and must parse as a type, catching typos) but isn't checked for equality against the field.
• ProtoList's generated clear reinitializes via Default::default() rather than retaining capacity — documented trade-off, acceptable per the trait's "where the underlying type allows" contract.
• MapStorage's Default/FromIterator are required by the message codec, not by this derive (a derived impl would wrongly force K: Default/V: Default) — must be hand-written, same reasoning as ProtoList.

Testing

Integration tests against real foreign types (ecow::EcoString, smallvec::SmallVec, smallbox::SmallBox, indexmap::IndexMap, plus hand-rolled types with non-conventional method names to exercise the override paths), covering both tuple-struct and named-field newtype shapes, decode/encode round-trips, and generic element types without Eq/Ord.

task lint and task test pass clean across the workspace.

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