Add atomic stack render command

[konojunya]

背景

Task 18 completes the initial native CLI surface by exposing deterministic standalone SVG rendering through the same pinned Stack Engine used by check and fmt.

実装方法

• add stack render <FILE> for SVG-only standard output
• add stack render <FILE> -o <OUTPUT> for same-directory atomic file output
• preserve existing output permissions and support atomic creation of new output files
• keep all diagnostics on stderr and preserve SVG for warning-only source
• return exit 1 without creating or replacing output for Stack errors
• return exit 2 for argument, input, output, stream, or engine operational failures
• reject identical input and output paths
• reuse the bounded, exclusive temporary-file writer shared with stack fmt

テスト方法

• cargo +stable fmt --check
• cargo +stable test --locked
• pinned specification formatter conformance
• cargo +stable clippy --all-targets --all-features --locked -- -D warnings
• RUSTDOCFLAGS="-D warnings" cargo +stable doc --no-deps --locked
• coverage: 91.10% regions, 98.48% functions, 93.12% lines
• stable release build and command help/version smoke
• Python standard-library XML parse of release-binary SVG
• Rust 1.85 tests and Clippy
• process tests for native-engine byte equality, warning output, existing/new atomic files, compiler-error no-output, missing paths, and read-only output cleanup

Review 視点 / 懸念点

• confirm stdout remains an SVG-only machine-readable stream
• confirm an existing output must remain unchanged on source or host failure
• atomic replacement currently relies on same-directory rename, matching the initial macOS/Linux delivery scope