chore: raise MSRV and toolchain to 1.85

[sksat (sksat)]

What

Raise the MSRV and the pinned dev/CI toolchain from 1.78 to 1.85, keeping them coupled:

file	before	after
Cargo.toml rust-version	1.78	1.85
rust-toolchain channel	1.78.0	1.85.0

The msrv job reads rust-version from Cargo.toml and the other jobs read rust-toolchain, so CI follows automatically.

Why

1.85 stabilized the 2024 edition (released 2025-02-20), and parts of the ecosystem now ship manifests that require it — Cargo 1.78 cannot even parse them:

• url >= 2.5.1 → idna 1.x → idna_adapter (edition2024) — blocks chore(deps): update rust crate url to v2.5.8 #121
• rmp-serde 1.x (edition2024 manifest) — blocks chore(deps): update rust crate rmp-serde to v1.3.1 #222

Staying on 1.78 meant those dependency updates fail CI structurally (feature edition2024 is required), and the lockfile ages while Renovate keeps retrying. 1.85 is ~16 months old at this point, which is still a conservative floor.

Verification

• cargo check --workspace --all-targets --locked passes locally under cargo 1.85.0 (same invocation as the msrv job).
• The publish dry-run job is unaffected (runs on stable since ci: run publish dry-run on stable, not the MSRV toolchain #209).

Follow-ups (separate PRs)

• Rebase/retry chore(deps): update rust crate url to v2.5.8 #121 (url) and chore(deps): update rust crate rmp-serde to v1.3.1 #222 (rmp-serde) — both should turn green on 1.85.
• The proptest cap comment in Cargo.toml references constraints from the 1.78 era and can be revisited.
• chore(deps): update dependency rust to v1.96.0 #116 (toolchain → 1.96) remains open if we later want to decouple the dev toolchain from the MSRV floor.

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