chore(rust): pin Rust toolchain to 1.95.0 via rust-toolchain.toml

[w-martin]

Prevents surprise CI failures from Rust stable updates — the class of breakage in #20.

Summary

• Adds rust-toolchain.toml at the repo root pinning Rust 1.95.0 with clippy and rustfmt components
• cargo, rustup, and dtolnay/rust-toolchain all respect this file automatically — local dev and CI stay in sync
• Future toolchain bumps require an explicit change to this file, making the update deliberate rather than silent

Updating the toolchain

Change channel = "1.95.0" to the target version, verify CI passes, merge.

Note

The dtolnay/rust-toolchain CI step still works as-is; the components: it specifies are superseded by the toolchain file. A follow-up can clean that up once the workflow OAuth scope is available.

Test plan

• rust-toolchain.toml exists at repo root
• cargo clippy -- -D warnings passes in rust/

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