Fix warning-denying builds on Rust 1.97

[Quik2007]

Problem

Building Vello with Rust 1.97 and -D warnings fails. Six functions use #[expect(single_use_lifetimes)] for a compiler false positive fixed in Rust 1.97. Because the lint no longer fires on Rust 1.97, the expectation is unfulfilled and becomes an error:

error: this lint expectation is unfulfilled
  = note: `-D unfulfilled-lint-expectations` implied by `-D warnings`

This affects Rust 1.97 and newer when warnings are denied. Vello's MSRV is Rust 1.89 and its pinned stable CI toolchain is Rust 1.95, so the existing CI jobs did not expose the failure.

Approach

Remove the source-level expectations and keep the old-compiler workaround only in the MSRV CI jobs. This follows the approach suggested in review: Rust 1.89 still gets -A single-use-lifetimes, while Rust 1.97 no longer sees an obsolete expectation.

The CI allowance can be removed when Vello's MSRV reaches Rust 1.97. Downstream projects that explicitly enable single_use_lifetimes while using Rust 1.89 through 1.96 may still need the same allowance.

Changes

• Remove six #[expect(single_use_lifetimes)] attributes from vello and vello_encoding.
• Set RUSTFLAGS=-A single-use-lifetimes in the native and Wasm MSRV jobs.
• Add the removal condition next to RUST_MIN_VER.

Testing

I reproduced the original failure on Rust 1.97.1, then checked the revised vello and vello_encoding crates with all features and the lockfile:

Toolchain	Configuration	Result
Rust 1.89.0	MSRV CI allowance	pass
Rust 1.89.0	-D warnings	pass
Rust 1.95.0	-D warnings	pass
Rust 1.96.0	-D warnings	pass
Rust 1.97.1	-D warnings	pass

Additional checks:

• warning-denying Clippy for vello and vello_encoding on Rust 1.95
• cargo test -p vello_encoding --lib --all-features --locked (31 passed)
• cargo test -p vello --lib --all-features --locked
• repository formatting and diff checks
• the full cross-platform CI matrix

AI usage: This PR was prepared with assistance from an AI coding tool.

[DJMcNab]

I believe that the fix we've used in previous cases is to add something like -A single-use-lifetimes to our MSRV checking jobs. We would then add a comment next to the MSRV saying to remove that once our MSRV is past the stable which fixes it (i.e. once it's past 1.97?)

Then we should remove the expects entirely.

[Quik2007]

Implemented your approach