Add atomic stack fmt command

[konojunya]

背景

Task 17 adds the formatter surface to the private native CLI while preserving the engine as the only implementation of Stack formatting semantics.

実装方法

• add stack fmt <FILE>, stack fmt --check <FILE>, and stack fmt -
• read and format bytes through the pinned Stack Engine API
• replace changed files with an exclusively created temporary file in the same directory, preserving permissions and syncing before rename
• skip replacement entirely when canonical bytes already match
• keep syntax, encoding, diagnostic-stream, and host I/O failures from changing the original file
• format syntactically valid semantic-error source while preserving diagnostics and exit 1
• pin the canonical formatter suite to specification SHA f382069928c805fe69b7a192bfd6a877036bc036

テスト方法

• cargo +stable fmt --check
• cargo +stable test --locked
• pinned specification formatter conformance through the CLI
• cargo +stable clippy --all-targets --all-features --locked -- -D warnings
• RUSTDOCFLAGS="-D warnings" cargo +stable doc --no-deps --locked
• coverage: 91.23% regions, 98.11% functions, 93.04% lines
• stable release build and command metadata smoke
• Rust 1.85 tests and Clippy
• process tests for comment preservation, idempotence, semantic errors, syntax no-write, stdin/stdout, check-only, permission preservation, unchanged inode, and atomic I/O failure cleanup

Review 視点 / 懸念点

• confirm semantic-error source should be formatted while still returning exit 1
• confirm - is the intended explicit stdin/stdout spelling
• atomic replacement currently relies on same-directory rename, matching the initial macOS/Linux delivery scope