Add an execution platform for the Canonical conformance suite

[konojunya]

Background

Duplicating specification example fixtures in the compiler repository may cause the canonical language contract and compiler tests to be updated independently. We directly execute the normalized IR / diagnostic contract and conformance suite published by stack-sh/specification, and reproducibly record the supported specification for each compiler revision.

Depends on stack-sh/specification#1.

Implementation Details

• Pinned the verified specification commit in tests/specification-revision.
• Added a feature-gated conformance integration test that compares the native IR for valid cases and portable diagnostics for invalid cases as semantic JSON.
• Checkout the pinned specification revision in CI and run the canonical suite separately from regular tests.
• Removed the canonical fixtures that were duplicated in the compiler repository, leaving only basic operations of the public API in inline integration tests.
• Restricted serde_json to dev dependencies only, avoiding any increase in runtime dependencies for the compiler library.
• Included Cargo dependencies in weekly Dependabot updates.

Testing Details

• cargo +stable fmt --check
• cargo +stable test
• STACK_SPECIFICATION_DIR=../specification cargo +stable test --features conformance --test conformance
• 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 --summary-only
• cargo +1.85.0 test
• cargo +1.85.0 clippy --all-targets --all-features -- -D warnings

Coverage result: regions 97.71%, functions 98.86%, lines 98.62%.

Review Points / Concerns

• Prioritizing CI reproducibility by pinning the specification to an exact commit rather than following the default branch
• Keeping JSON mapping enclosed in a test-only adapter without locking down native Rust APIs or runtime dependencies
• After the specification PR is squashed and merged, tests/specification-revision needs to be updated to that SHA once the merge commit is finalized
• When expanding the canonical suite, compiler support and the revision pin update should be reviewed together in the same PR