MSRV: step rust-version toward current stable as features land (1.70 -> ~1.98)

[JSBtechnologies]

atrium declares rust-version = "1.70" (June 2023). Current stable is 1.98.1 (2026-09-03). The founder's direction (2026-09-30): move toward the nearest stable, gradually, as features land, not as a standalone big-bang bump.

How

• One step per feature pass. When a feature would benefit from a newer language or std feature, raise rust-version to the release that has it, in the same change.
• Each step: bump rust-version in Cargo.toml, rustup toolchain install <ver>, then run python dev.py check. The gate reads the MSRV from Cargo.toml and builds on it (cargo +<ver> check --all-targets --locked). Take the clippy suggestions the new floor unlocks, and add a line to the changelog.
• Target: within about two stable releases of current. An app has no downstream crates pinned to it, so it can stay close to stable.

Likely steps (what each one unlocks)

• 1.74: Cargo reads the [lints] table. Today 1.70 ignores it with a warning.
• 1.81: #[expect] and reason = "..." on lint attributes. This unblocks allow_attributes_without_reason, which was deferred in d33a784.
• 1.85: edition 2024.
• 1.88: let chains, which need edition 2024.
• Current stable: whatever the feature at hand needs.

Scope note

The nativelite-* library crates also declare 1.70. They are separate decisions: a library's MSRV limits every downstream user, so the libraries can stay lower than atrium. Raising atrium doesn't require raising them.