Bound stream Send impls on the handle's Target, not the handle

[cf-rhett]

Summary

The four stream types all store Q::Target, but their Send impls were written against Q:

unsafe impl<Q: BbqHandle + Send> Send for StreamProducer<Q> {}

BbqHandle is a public, unsealed, safe trait with an associated Target, so a downstream impl is free to pair a Send handle with a !Send target. Q: Send then says nothing about the thing actually being stored, and you get a Send producer holding an Rc. framed.rs already gets this right — FramedGrantW and FramedGrantR are bounded on Q::Target: Send — so the two modules disagreed on the same question.

StreamProducer and StreamConsumer hold nothing but Q::Target. They never needed a manual impl; the derived one is already exactly right, so this deletes them. StreamGrantW and StreamGrantR also hold a NonNull, so they keep an unsafe impl, now bounded on Q::Target: Send to match framed.rs.

Only in-tree BbqHandle impls are &BBQueue and Arc<BBQueue>, both of which use type Target = Self — so nothing in the crate or its tests changes behavior, and the full matrix below still passes.

Reproducer

A purely safe downstream impl was enough to forge the bad bound. Against main this compiles and passes; with this change it fails to compile with Rc<BBQueue<..>> cannot be sent between threads safely.

#[derive(Clone, Copy)]
struct SendyHandle;                 // ZST, so Send

thread_local! {
    static RING: Rc<Ring> = Rc::new(BBQueue::new());
}

impl BbqHandle for SendyHandle {
    type Target = Rc<Ring>;         // but the Target is not
    /* ... */
    fn bbq_ref(&self) -> Self::Target { RING.with(|r| r.clone()) }
}

assert_send::<StreamProducer<SendyHandle>>();   // compiles on main

I left this out of the tree since catching it needs a compile-fail harness (trybuild) that the crate does not currently pull in. Happy to add it if you would like the coverage.

Testing

• cargo test --features=std
• cargo build --no-default-features
• cargo build --features=std
• cargo build --no-default-features --target=thumbv6m-none-eabi
• cargo build --target=thumbv6m-none-eabi --features=portable-atomic-unsafe-assume-single-core
• ./miri.sh

Note on my other open PRs

Independent of #132 and #133 in substance. #133 merges with this cleanly. #132 collides textually only — it appends a test module directly below the StreamGrantR impl this rewrites, so whichever lands second wants a keep-both resolution rather than a semantic one.