feat(repopo): Add Rust project structure policies

[tylerbutler]

Summary

Add policies for validating Rust project file structure and build configuration.

Proposed Policies

RustToolchainExists

Require rust-toolchain.toml for reproducible builds:

makePolicy(RustToolchainExists, {
  requiredComponents: ["rustfmt", "clippy"],
  channel: "stable", // or allow "nightly"
})

RustfmtConfigExists

Require rustfmt.toml for consistent formatting:

makePolicy(RustfmtConfigExists, {
  requiredSettings: {
    edition: "2021",
  },
})

CrateStructureValid

Validate crate structure matches Cargo.toml:

• If src/lib.rs exists, validate it's configured as a library
• If src/main.rs exists, validate binary configuration
• Ensure [[bin]] entries have corresponding files

CargoLockPolicy

Context-aware Cargo.lock validation:

makePolicy(CargoLockPolicy, {
  binaries: "require",    // Binaries should commit Cargo.lock
  libraries: "warn",      // Libraries typically shouldn't
})

NoTargetArtifacts

Prevent accidental commits of build artifacts:

• Detect .rlib, .rmeta, .d, .so, .dylib files outside /target/
• Could extend NoLargeBinaryFiles or be standalone

Use Case

Ensuring Rust projects follow best practices for reproducible builds and proper crate organization.