Tie boxed future to the receiver lifetime for &self-only methods

[ashi009]

Follow-up to #297; works around rust-lang/rust#157595.

For a method whose only borrowed input is the receiver and which has no generic parameters, the boxed future is now tied to the receiver's lifetime and no 'async_trait is emitted:

// async fn m(&self) -> R  currently:
fn m<'life0, 'async_trait>(&'life0 self) -> Pin<Box<dyn Future<Output = R> + Send + 'async_trait>>
where 'life0: 'async_trait, Self: 'async_trait;
// with this change:
fn m<'life0>(&'life0 self) -> Pin<Box<dyn Future<Output = R> + Send + 'life0>>;

The two forms are semantically identical — the future still borrows self, neither is 'static — but the avoided outlives bounds defeat the trait solver's global Send/Sync cache, re-proving the captured state once per impl. One trait, 300 &self-only impls over a deep Arc<Mutex<Vec<…>>> state, rustc 1.96.0:

	evaluate_obligation
async-trait 0.1.89	18.07 s
this PR	77.7 ms

• transform_sig skips the 'async_trait param and its outlives bounds for eligible signatures and reuses the receiver's 'life0 on the future; the inferred Send/Sync bound for default methods is kept (Self: 'life0 is implied by &'life0 self)
• eligibility is decided from the signature alone — receiver (&self / &mut self / self: &Self) as the sole reference input, no generic/const params, no impl Trait arg, nothing naming 'async_trait (new NeedsAsyncTrait visitor) — because the trait and its impls expand in separate invocations and must agree on whether 'life0 is late-bound
• new region_free_receiver_lifetime test pins the eligible forms (&mut self, borrowing return, owned args, default body); tests/ui/lifetime-span.stderr re-blessed (spans shift on already-invalid code)

Limitation. The receiver lifetime becomes late-bound for eligible methods. The one observable break: a trait method written with where …: 'async_trait whose impl omits the clause now fails with E0195 (the two sides disagree on early/late-bound). Loud compile error, not UB. GitHub search finds no code with that combination: where Self: 'async_trait has zero hits, and the ~7 repos using where T: 'async_trait all pair it with shapes the gate already excludes.