Introduce `might_be_async` as a workspace dependency, add it to
`mingling_core`, and regenerate all Cargo.lock files accordingly.
Multiple examples and test packages also pick up a newly transitive
`equivalent` crate.

Replace the `builds` feature with `build` across the crate, keeping
a backward-compatible alias for the old name.  Update all internal
visibility from `#[doc(hidden)] pub mod` to `pub(crate) mod` and
reorganize the `__private` module into a dedicated `private.rs`.

Move file I/O and AST parsing into spawn_blocking closures to avoid
blocking the async runtime, enabling concurrent processing of multiple
Rust source files.

Add `mling` wrapper binary that delegates to `mingling-cli`, enabling
separate binary names while maintaining Cargo conventions.