feat(rust): async-spawn - build under no_std + alloc

[mattwilkinsonn]

Summary

Closes #1599.

Follow-up to #1591, which made the async feature work under no_std. That PR left std on the async-spawn feature, but nothing in src/rt/async_support/spawn.rs actually requires std — it's Box, Vec, Future, Context, Poll, and FuturesUnordered / StreamExt from the futures crate. All of these are available through core / alloc once futures opts out of its default std feature.

This PR is the mechanical swap: std::* → alloc::* / core::*, drop std from the feature definition, and add default-features = false, features = ["alloc"] to the futures dep.

Changes

Five edits across three files:

• crates/guest-rust/Cargo.toml:
  • async-spawn = ['async', 'dep:futures', 'std'] → async-spawn = ['async', 'dep:futures']
  • futures = { version = "0.3.30", optional = true } → futures = { version = "0.3.30", optional = true, default-features = false, features = ["alloc"] }
• crates/guest-rust/src/rt/async_support/spawn.rs:
  • use std::{boxed, future, task, vec}; swapped for use alloc::{boxed, vec}; + use core::{future, task};
• .github/workflows/main.yml:
  • rustup target add wasm32v1-none in the check job setup
  • Two cargo build --target wasm32v1-none -p wit-bindgen --no-default-features --features {async,async-spawn} lines so a regression that pulls std back in fails CI instead of silently relinking against wasm32-wasip1's std.

wasm32v1-none (Tier 2, stable) is used for the guard because it has no std at all, unlike wasm32-unknown-unknown which happily links std even when you opt out via features.

Verification

Default-features build is unchanged. The async-spawn feature also builds without std:

$ cargo check -p wit-bindgen --no-default-features --features "realloc,async,async-spawn,bitflags,macros"
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 1.70s

The new CI guards are correct — on main (before this PR) the wasm32v1-none line fails to link because futures-executor pulls in std:

$ cargo build --target wasm32v1-none -p wit-bindgen --no-default-features --features async-spawn
error[E0463]: can't find crate for `std`
   --> memchr-2.8.0/src/lib.rs:198:1
    |
198 | extern crate std;
    | ^^^^^^^^^^^^^^^^^ can't find crate

With the patch applied, it compiles clean.

Downstream verification: my WIP project (not public yet) carries this exact patch on top of 0.55.0 and builds its agent as a fully no_std component on wasm32-wasip3 with -Zbuild-std=core,alloc.