Suppress the documented Crossbeam queue-buffer race in ThreadSanitizer.

[samuel-williams-shopify]

Load narrowly scoped ThreadSanitizer suppressions for Crossbeam's speculative queue-buffer reads and writes.

The ThreadSanitizer job on main reports a race between Worker::push and Stealer::steal_batch_and_pop. Crossbeam intentionally reads queue slots before atomic validation and discards speculative values when validation fails. Its volatile buffer accesses remain a known Rust memory-model limitation, discussed in Crossbeam #589.

Add .github/tsan-suppressions.txt, matching only crossbeam_deque::deque::Buffer::read and Buffer::write with generic type wildcards, and load it through TSAN_OPTIONS in the sanitizer job. Document the accepted limitation and the need to reassess the patterns when updating Crossbeam. Race detection remains enabled for reports outside the matched buffer accesses.

Because the change intentionally suppresses a sanitizer report, a temporary ThreadSanitizer probe checked both outcomes: matching buffer access reports are suppressed, and an unrelated race still produces a failing exit status. The Linux sanitizer job exercises the actual Rust executor with these patterns.