revert: keep the MSRV at Rust 1.75 #44

Problem

#43 raised the MSRV from Rust 1.75 to Rust 1.84 without the maintainer's decision.
Only the maintainer moves the MSRV.

Proposed behavior

• Revert build!: require Rust 1.84 and let lint exceptions expire #43, so the crate requires Rust 1.75 again.
• The three lint exceptions return to the registry in xtask/src/gates.rs, as ADR 0013 describes.
• .github/renovate.json stays, so Renovate still can't move the MSRV toolchain in CI.

Alternatives considered

• Rust 1.75 lacks #[expect] and pointer addr, so the three exceptions can't stay closed on it.

Compatibility and platform scope

Projects on Rust 1.75 through 1.83 can use 0.2 again.
The nine setters that #43 made const fn lose const, and no release shipped them.
Windows behavior doesn't change.

Acceptance criteria

• The tree matches main before build!: require Rust 1.84 and let lint exceptions expire #43, except for .github/renovate.json.
• CI checks Rust 1.75 where it checked Rust 1.84.