remote-derive: optional as_shared override on derive(ProtoBytes)

[iainmcgin]

Follow-up to #284, stacked on iain/vectored-encode (it needs ProtoBytes::as_shared, which only exists there). Retarget to main if #284 lands first.

Problem

buffa-remote-derive (#251) generates the newtype's only permitted ProtoBytes impl, and that impl inherits the trait's as_shared default of None. So a remote bytes newtype — even one wrapping bytes::Bytes — always takes the copy path into a segmented (Rope) sink, and the user cannot override it: a manual impl alongside the derive is a coherence error (E0119), and the #[buffa(...)] grammar had no key for it. Output bytes are identical either way, so the missed zero-copy is silent.

Change

#[buffa(remote = ..., as_shared = path)] now generates the hook:

#[derive(Clone, PartialEq, Default, Debug, buffa_remote_derive::ProtoBytes)]
#[buffa(remote = bytes::Bytes, as_shared = share::handle)]
pub struct SharedBlob(pub bytes::Bytes);

mod share {
    pub fn handle(b: &bytes::Bytes) -> Option<bytes::Bytes> {
        Some(b.clone())
    }
}

The path is called as a free function on the wrapped field (path(&self.0) / path(&self.field)), shape fn(&Remote) -> Option<Bytes>; a signature mismatch is a type error at the generated call site. Absent the key, nothing is generated and the trait default applies — existing derives produce byte-identical output. This mirrors the existing parse_with_overrides mechanism used by ProtoBox/MapStorage, in a second mode: an opt-in hook with no default rather than a renamed inherent-method default (the helper's doc now describes both modes).

Testing

• Unit tests: default stays None; override returns the wrapped handle (pointer equality); end-to-end Rope splice through put_shared_bytes_field (payload segment pointer-equal to the source Bytes, byte parity vs a contiguous sink; payload sized 2 * DEFAULT_MIN_SEGMENT so the splice threshold can move without breaking the test); named-field newtype shape.
• Driven end-to-end from a scratch consumer: bytes_type_custom message encode into a Rope splices zero-copy; typo'd key and wrong-signature callable both produce legible errors.

[github-actions]

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