build: migrate workspace to `resolver = "3"` for MSRV-aware resolution

[mxaddict]

Rust workspace currently sets resolver = "2". Edition 2024 / Rust 1.84+ ships resolver = "3", which adds MSRV-aware version resolution: dependency resolution will skip transitive crate versions whose rust-version exceeds the workspace MSRV, preventing accidental MSRV breakage from a routine cargo update.

Change

 [workspace]
-resolver = "2"
+resolver = "3"

Why

• Other kryptic-sh repos (krypt) already on resolver = "3". Aligns convention.
• cargo update on a workspace pinned to a low MSRV no longer pulls in newer dep versions that silently bump MSRV.
• No-op for builds that already satisfy the highest dep MSRV in the lockfile.

Caveats

• Requires Rust 1.84+ at build time (kryptic-sh CI already past this).
• Does not retroactively scan the existing Cargo.lock; first effect is on next cargo update.

Refs

• Cargo book: https://doc.rust-lang.org/cargo/reference/resolver.html#resolver-versions
• RFC 3537: https://rust-lang.github.io/rfcs/3537-msrv-resolver.html