pin Rust toolchain to 1.95.0 for deterministic rustfmt

[johncarmack1984]

Summary

Adds rust-toolchain.toml pinning to Rust 1.95.0 (rustfmt 1.9.0-stable, released 2026-04-14).

Why

dprint's exec plugin shells out to rustfmt --edition 2024 for .rs files. Without a pinned toolchain, both CI (ubuntu-latest's bundled Rust) and contributors run whatever rustfmt is on PATH. Different rustfmt builds emit different layouts for the same input -- e.g., let-chain if let Some(_) = expr { ... } gets formatted as one line by some versions and two lines (brace on its own line) by others. That surfaced in #22 as a dprint check failure on taurpc/src/export.rs even though the contributor's local rustfmt was happy with their formatting.

The --edition 2024 flag stays as-is (it's the latest stabilized Rust language edition; there is no 2025 edition). What this pin closes is the binary-version drift, separate from the edition flag.

Verified

• rustup run 1.95.0 rustfmt --edition 2024 --check taurpc/src/export.rs -> zero diff on current main
• pnpm lint clean (types + format + js)

Caveat for #22 specifically

This makes the toolchain deterministic going forward. It does not retroactively reformat files. After rebasing #22 on this main:

• The taurpc/src/export.rs content Oscar wrote uses a layout produced by a different rustfmt version and will still fail dprint check until he runs pnpm format (or cargo fmt) locally with the pinned 1.95.0 toolchain and pushes the resulting diff
• Any other in-flight branches with rustfmt-formatted Rust changes will pay the same one-time reformat cost