fix(ci): stabilize allocation probe on macOS MSRV

[jcwal1516]

Summary

• allocate the exact-count probe directly as boxed slice storage
• keep the Rayon pool uninitialized while allocator self-tests run
• preserve the exact one-allocation and one-deallocation assertions

Root cause

The post-merge full-validation run failed only on macOS with Rust 1.96. In a debug build, Box::new on the large inline array could introduce an additional allocator-backed temporary. The eager Rayon pool broadcast could also contribute unrelated worker activity to the intentionally process-wide meter, making the exact-count self-test nondeterministic.

Validation

• reproduced the original failure with cargo +1.96 test -p j2k-alloc-probe --test allocation_probe
• passed 20 consecutive focused Rust 1.96 runs
• passed the focused test under Rust 1.96 with CARGO_PROFILE_DEV_DEBUG=full
• passed the focused test under stable Rust with CARGO_PROFILE_DEV_DEBUG=full
• cargo xtask fmt
• cargo xtask clippy
• cargo xtask clippy-strict
• cargo xtask test

Failed run: https://github.com/frames-sg/j2k/actions/runs/30873033592/job/91878736880