chore: pin Rust toolchain to 1.96.0

[apriendeau]

Summary

Pins the project to Rust 1.96.0 by adding a rust-toolchain.toml:

[toolchain]
channel = "1.96.0"
components = ["clippy", "rustfmt"]
profile = "minimal"

Previously nothing pinned a version — no rust-toolchain.toml, no MSRV in Cargo.toml, and CI just installed stable. This ensures all developers and tooling use a consistent toolchain.

Verification

• rustup show confirms 1.96.0 is overridden by the toolchain file, with clippy + rustfmt present.
• just check passes clean: fmt, clippy (-D warnings), build, all tests, and cargo check --locked.

Out of scope (intentionally)

• CI still uses stable — the workflows run rustup default stable, which overrides the toolchain file. CI won't actually use 1.96.0 until those lines change.
• No MSRV (rust-version) declared in Cargo.toml.

⚠️ Note: the CI version-check job requires Cargo.toml version to be bumped from main. This PR doesn't change the version, so that check will fail until the version is bumped (or the check is waived).

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