sdk: make the IntentBody derive no_std (emit ::core/::alloc paths)

[mfw78]

Part of #137. Flagged by the #138 sweep.

Problem. The IntentBody derive in crates/nexum-macros/src/intent_body.rs emits ::std paths, so any consumer (e.g. crates/cow-venue) links std and cannot be truly no_std (documented as a constraint in crates/cow-venue/src/lib.rs). Harmless for wasm32-wasip2 guests (wasip2 is a std target) but breaks the no_std contract the venue body slice is meant to honour.

Scope. Change the derive to emit ::core/::alloc paths so #[derive(IntentBody)] consumers can be #![no_std].

Acceptance. cow-venue and a fresh #![no_std] probe build --no-default-features without linking std.

[mfw78]

Landed via #451 (squash-merged into dev/m1). Closing manually since the merge target is not the default branch.