Added a Stack compiler frontend written in Rust

[konojunya]

Background

The purpose is to translate the canonical specification of the Stack language into an executable compiler frontend and prepare a pure Rust core that can be shared across browsers, CLI, and hosted services. The renderer, theme/icon resolution, layout, and HTTP are out of scope for this change.

Implementation Details

• Implemented the lexer, recursive-descent parser, source AST, semantic validation, and normalized IR using Rust 2024 with no runtime dependencies.
• Documented three ADRs covering the approach for the Rust core, AST/IR separation, and handwritten parser.
• Implemented source spans and STK diagnostics, node/group/edge/layout/theme constraints for Stack 1.0, defaults, and complexity limits.
• Banned panic-inducing operations such as unwrap and expect using Clippy.
• Separated CI into main CI using the latest stable Rust/Cargo and MSRV CI using Rust 1.85.
• Added actions/checkout v7, weekly Dependabot, and a 95% unit coverage gate.
• Renderer, server, and theme/icon resolution remain unimplemented.

Testing Details

• cargo +stable fmt --check
• cargo +stable test
• cargo +stable clippy --all-targets --all-features -- -D warnings
• RUSTDOCFLAGS="-D warnings" cargo +stable doc --no-deps
• cargo +stable llvm-cov --lib --all-features --workspace --fail-under-lines 95 --fail-under-functions 95 --fail-under-regions 95
• cargo +1.85.0 test
• cargo +1.85.0 clippy --all-targets --all-features -- -D warnings
• Byte-level diff with the 4 examples from the canonical specification
• 30 unit tests + 5 integration tests
• Latest stable coverage: lines 98.62%, functions 98.86%, regions 97.71%

Review Points / Concerns

• Is the separation of concerns between the source-oriented AST and normalized IR suitable for the future WASM/CLI boundary?
• Is the diagnostic policy of stopping at the first syntax error while collecting semantic errors appropriate?
• Is the policy of maintaining Rust 1.85 as the MSRV while using the latest stable as the main CI appropriate?
• Are the boundaries appropriate regarding excluding theme/icon resolution, layout, and the SVG renderer from the compiler repository?