docs: record the reason for the Rust 1.84 MSRV #45

Problem

ADR 0018 sets the MSRV to Rust 1.84 but doesn't say why it may exceed the MSRV of the Windows bindings.
windows-sys 0.61, the crate's only Windows dependency, requires Rust 1.71, and the windows crate 0.62 requires Rust 1.82.
The ADR also doesn't say who moves the MSRV, and #43 moved it without the maintainer's decision.

Proposed behavior

• ADR 0018 calls windows-spawn a convenience layer over these bindings, so a project that must build with an older Rust can call windows-sys directly.
• ADR 0018 states that only the maintainer moves the MSRV.
• ADR 0018 rejects Rust 1.82, the MSRV of the windows crate, and an MSRV that follows windows-sys.

Alternatives considered

• revert: keep the MSRV at Rust 1.75 #44 proposed reverting build!: require Rust 1.84 and let lint exceptions expire #43, which would keep the lint exception registry for projects that can call windows-sys directly.

Compatibility and platform scope

The change touches documentation alone.

Acceptance criteria

• ADR 0018 records why the MSRV may exceed that of windows-sys and of the windows crate.
• ADR 0018 records that only the maintainer moves the MSRV.