Add Rust Cargo integration helpers

[YH390389859]

Summary

Closes #100.

This adds initial Rust Cargo integration helpers for Lux-managed Rust projects:

• add Lux.Rust and Lux.Rust.Cargo helpers
• generate Lux-managed Cargo.toml manifests
• safely patch dependency sections in existing manifests without rewriting unrelated Cargo.toml content
• inspect dependencies against Cargo.lock data and optional version indexes
• plan and run Cargo fetch/build/test commands
• generate Cargo cache metadata and stable cache keys
• include a minimal priv/rust scaffold and Rust Cargo guide

Notes

This intentionally does not try to replace Cargo's full dependency resolver or act as a complete TOML AST-preserving rewriter. The full manifest reader/writer is scoped to Lux-generated manifests, while the file-level dependency helpers patch only dependency sections to preserve workspace, target, binary, profile, and other unrelated manifest content.

Verification

• mise exec -- mix format --check-formatted lib/lux/rust.ex lib/lux/rust/cargo.ex test/unit/lux/rust/cargo_test.exs mix.exs guides/language_support.md guides/language_support/rust_cargo.md
• mise exec -- mix test test/unit/lux/rust/cargo_test.exs --include unit - 24 tests, 0 failures
• mise exec -- mix test - 1 doctest, 4 properties, 1299 tests, 0 failures, 1288 excluded
• mise exec rust@stable -- cargo check --manifest-path priv/rust/Cargo.toml
• git diff --check