build!: require Rust 1.84 and let lint exceptions expire

[P4suta]

Summary

Closes #42.

The crate now requires Rust 1.84, so its last three lint exceptions expire instead of sitting in a registry.

• #[expect(lint, reason = "…")] replaces the allow on sys and on a probe's process::exit, and the registry in xtask/src/gates.rs goes.
  Clippy's allow_attributes and allow_attributes_without_reason and rustc's unfulfilled_lint_expectations deny their cases, and the gate refuses every allow, inner ones included.
• handle_value and handle_from_value move to domain::conversion_rules and convert through addr and ptr::without_provenance_mut, so no as remains.
  Kani proves the round trip for every isize.
  With the byte offset from null, it refuted every symbolic negative value.
• Pointers to locals that Windows reads or writes use &raw mut and &raw const, as borrow_as_ptr requires under the new MSRV.
• Nine Command setters that only record a choice become const fn, and the public API snapshot records them.
• The workspace uses resolver 3, and CI, cargo xtask msrv, the README, CONTRIBUTING, the changelog, and the migration guide name Rust 1.84.
• .github/renovate.json keeps Renovate off the numeric Rust toolchain in CI, the MSRV pin.
  The organization's preset automerges minor updates, and the open build(deps): update all non-major dependencies #30 moves the MSRV check from Rust 1.75 to Rust 1.99.
• ADR 0018 records the decision, and ADR 0013 and 0016 no longer describe a registry.

Validation

• Clippy passed for x64, i686, and ARM64 Windows, with and without windows_spawn_vectors, and on macOS.
• Rust 1.84 checked x64 and i686 Windows.
• cargo xtask proofs kani verified every proof, including the handle round trip for every isize, and every negative control failed on its marker.
• cargo miri test passed the library tests.
• On Windows 11, the fmt, clippy, gates, test, doc, cross-targets, and linux-empty steps of cargo xtask passed.
  That machine has no Rust 1.84, so CI runs the MSRV step.
• cargo xtask public-api matches the updated snapshot.
• A probe crate showed that Clippy's two lints catch #[allow], its cfg_attr form, and #[expect] without a reason, but not an inner #![allow] with one.
  The gate's new test covers that form.
• A probe crate showed that Rust 1.84 still counts no use inside const _, so the two LAYOUT notes keep their reason with the new version.

Checklist

• The relevant local tests ran.
• Deterministic tests cover each behavior change.
• The docs state each new or changed ownership, safety, or cleanup contract.
• The public API gains const on nine setters, and the snapshot records it.
• The PR holds no secrets or public vulnerability details.
• The PR stays focused and updates the relevant documentation.