Add stack check command

[konojunya]

背景

Task 16 starts the native CLI surface with a validation command that delegates all language, theme, layout, and routing behavior to the merged Stack Engine implementation.

実装方法

• add the private Rust stack binary with Rust 1.85 as its minimum supported version
• pin stack-engine to merge SHA 07b71c783a3c1f9037d865b19672a9522eb0b24d
• implement stack check <FILE> using raw file bytes without modifying the source
• write ordered diagnostics to stderr while keeping stdout empty
• use exit 0 for valid or warning-only source, exit 1 for Stack errors, and exit 2 for argument, host I/O, or operational failures
• upgrade CI from foundation checks to stable/MSRV Rust gates, coverage, release build, help, and version smoke tests

テスト方法

• cargo +stable fmt --check
• cargo +stable test --locked
• cargo +stable clippy --all-targets --locked -- -D warnings
• RUSTDOCFLAGS="-D warnings" cargo +stable doc --no-deps --locked
• cargo +stable llvm-cov --all-features --locked --fail-under-lines 90 --fail-under-functions 95 --fail-under-regions 90
• cargo +stable build --release --locked
• cargo +1.85.0 test --locked
• cargo +1.85.0 clippy --all-targets --locked -- -D warnings
• release binary --help and --version smoke tests
• process tests for valid, warning-only, invalid UTF-8, and missing-file inputs

Review 視点 / 懸念点

• confirm the diagnostic text and exit-code contract is suitable for both human use and CI
• confirm the CLI remains only a host adapter and does not duplicate engine behavior
• external binary distribution remains blocked on the existing licensing checklist