Stabilize cross-platform ABI contract tests

[LunaStev]

Summary

• accept Clang pointer-aggregate IR spellings that are ABI-equivalent on x86_64 and AArch64 while keeping Wave ABI assertions strict
• generate the RISC-V archive fixture with pinned LLVM ar in Darwin format instead of relying on host ar behavior
• include the target and accepted contracts in ABI assertion diagnostics

Validation

• cargo fmt --all --check
• cargo check --locked --jobs 2
• cargo clippy --locked --all-targets --jobs 2 -- -D warnings
• RUSTDOCFLAGS=-D warnings cargo doc --locked --no-deps --jobs 2
• cargo test --locked --all-targets --jobs 2
• Python tooling compilation and unit tests
• standard library policy check
• release build and Wave corpus: 92 passed
• clean-HOME end-to-end suite: 100 passed, 8 environment/architecture skips, 0 failed