Fold invariant_context into context

[coord-e]

#[thrust_macros::context] stamped each method of an impl/trait with the enclosing header, while #[thrust_macros::invariant_context] threaded the same header — plus the host signature — into the invariant! calls in a function body. Two attributes for one question: what does the code inside this item see?

They become one. #[thrust_macros::context] now takes a function as well, and an impl/trait hands each of its methods the enclosing header along with the attribute itself, so a method carrying a loop invariant no longer needs one of its own:

#[thrust_macros::context]
impl Counter {
    fn run(&mut self) -> i64 {
        let init = *self;
        while rand() {
            thrust_macros::invariant!(|init: Self, self: &mut Self| init.value <= (*self).value);
            self.value += 1;
        }
        init.value
    }
}

A free function still opts in explicitly, with #[thrust_macros::context] in place of the old name.

Who does what

The impl/trait expansion stays a distributor: it knows which items are methods and what header they sit under, and nothing about what a body holds. It stamps two attributes per method —

#[thrust::_outer_context(impl Counter)]
#[::thrust_macros::context]
fn run(&mut self) -> i64 { .. }

— and each method's body is then threaded by its own expansion of the attribute, the same one a free function gets, which reads the header back from #[thrust::_outer_context(..)]: the path requires/ensures already take. That expansion accepts a free function, an impl method, or a trait method through the existing FnItemWithSignature, so a trait method with no body needs no special case.

Not extending a where clause nothing asked for

Threading a body that names no spec macro now leaves it alone. invariant_context extended the host's where clause with the Model predicates unconditionally, which was fine while it was opt-in per function; with every method of a #[context] item threaded, those bounds would otherwise land on methods that have no formula to justify them.

Changes

context.rs	dispatches on the item: a function is threaded, an impl/trait hands its methods the header and this attribute. ContextInjector moves here from invariant_context.rs, which is deleted
spec.rs	FnItemWithSignature::sig_mut, to extend the threaded function's where clause
tests/ui	17 files: free functions take #[thrust_macros::context], methods drop the attribute entirely

No behaviour change for the analyzer; the macro expansion each test produces is the same as before.