test(tbtc/signer): pin concrete coordinator vector in order-independence test

[mswilkison]

Stacked on #4005 (base: extraction/frost-signer-mirror-2026-05-26). Companion to #4026 on the Go branch.

What

select_coordinator_is_input_order_independent in pkg/tbtc/signer/src/go_math_rand.rs asserted only that two input orderings of the member set agree — not what they agree on. This pins the concrete result (Some(4) for (members=[1..6], seed=333, attempt=4)), matching the value now pinned on the Go side in TestSelectCoordinator_CrossLanguagePinnedVectors (#4026).

Why

Coordinator selection must agree byte-for-byte between Go's SelectCoordinator and this Rust port of Go's math/rand shuffle, or honest nodes elect different coordinators and the ROAST liveness path fractures. With the vector sets symmetric across both test suites, either implementation drifting fails its own unit tests instead of surfacing in mixed-version soak testing.

Verified locally: cargo test --lib go_math_rand passes, cargo fmt --check clean.

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