Pin the expanded diagnostic conformance suite

[konojunya]

Background

The canonical specification now covers all 22 Stack 1.0 diagnostics emitted by compiler stages 1 through 4. The compiler must pin the final squash commit from stack-sh/specification#2 before claiming conformance with that expanded suite.

Implementation

• Update tests/specification-revision to 8a4cf2ec97f5fba702ba3f27388fcab31f039ef5.
• Keep compiler behavior unchanged; the existing conformance adapter already matches every new code, severity, end-exclusive range, and diagnostic ordering expectation.

Testing

• 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
• cargo +1.85.0 test
• cargo +1.85.0 clippy --all-targets --all-features -- -D warnings
• Coverage: 97.71% regions, 98.86% functions, 98.62% lines

Review focus / concerns

Confirm that the pinned revision is the final squash commit from specification PR #2. This PR changes no parser, validation, IR, or diagnostic implementation.