Support Self and generic variables in loop invariants

[coord-e]

Summary

Extends thrust_macros::invariant! to invariants over generic- and Self-typed variables, by extending #[thrust_macros::context] to thread the enclosing context into each invariant!.

#[thrust_macros::context]
fn f<T: PartialEq>(v: T) {
    while cond {
        thrust_macros::invariant!(|x: T, v: T| x == v);
        ...
    }
}

Frontend (thrust-macros)

• #[thrust_macros::context] (which already stamps the enclosing impl/trait header onto methods so method-level requires/ensures recover the outer generics) now also accepts free functions and modules and threads the in-scope generics, where-predicates, and (in methods) Self into every invariant! it encounters.
• invariant! accepts a threaded [self] [params] [wheres] <closure> input. The generated formula function re-declares the threaded generics (shadowing the enclosing ones) and is instantiated via turbofish; in methods, Self is re-declared the same way as a synthetic type parameter and instantiated with the real Self (legal in expression position).
• The concrete, non-generic form from Implement loop invariant annotation #95 keeps working without context.
• The macro crate is reorganized into context, invariant, and spec modules; the root keeps only the proc-macro entry points and a few shared private helpers.

Test plan

• Pass/fail UI test pairs for the generic and Self cases (loop_invariant_generic, loop_invariant_self).
• cargo test --test ui (203 passed), cargo clippy --all-targets, cargo fmt --check all clean.

Notes

• A struct used as an invariant variable must model structurally: its Model::Ty must mirror its fields (e.g. a one-field tuple struct models as a one-tuple). A mismatching model trips an existing subtyping panic unrelated to this PR.

https://claude.ai/code/session_01WB28auaD8dSQrckqBwJWBt