Prepare stack-compiler for crates.io

[konojunya]

Summary

• declare crates.io as the only publication registry and add complete discovery metadata
• constrain the published source archive to the compiler source, tests, README, and Apache-2.0 license
• document the versioned Cargo dependency command
• make every CI run verify the exact crates.io package through Cargo's publish dry run

Testing

• cargo fmt --check
• cargo test --locked
• STACK_SPECIFICATION_DIR=../specification cargo test --features conformance --test conformance --locked
• cargo clippy --all-targets --all-features --locked -- -D warnings
• RUSTDOCFLAGS='-D warnings' cargo doc --no-deps --locked
• cargo publish --dry-run --locked --allow-dirty
• cargo +1.85.0 test --locked
• cargo +1.85.0 clippy --all-targets --all-features --locked -- -D warnings
• cargo llvm-cov --lib --all-features --workspace --fail-under-lines 95 --fail-under-functions 95 --fail-under-regions 95