#78 project-toolchain-and-rustfmt-config

Summary by Sourcery

Standardize the Rust toolchain and formatting policy across development and CI.

New Features:

• Add project-wide stable Rust toolchain and rustfmt configurations to align local development with CI.

Enhancements:

• Document formatting commands and clarify toolchain management and reproducibility expectations.
• Apply the configured formatting consistently across the Rust workspace.

Build:

• Pin the stable Rust channel with the rustfmt and clippy components via rust-toolchain.toml.

Documentation:

• Update contributor guidance with the project formatting policy and toolchain setup.

Summary by cubic

Adds project-wide rustfmt configuration and toolchain pinning so cargo fmt --check passes identically on every toolchain and local builds match CI.

• Adds rust-toolchain.toml pinning stable with rustfmt and clippy components, matching what CI installs.
• Adds rustfmt.toml with edition = "2021" and max_width = 100, restricted to options that behave the same across stable rustfmt releases.
• Reapplies formatting across 10 crates (whitespace only) per the new config, replacing reliance on a global ~/.rustfmt.toml that could fail on newer rustfmt.
• Updates AGENTS.md and rust.instructions.md to document the new formatting and toolchain setup.

Refs #77