feat(frost/roast): RFC-21 Phase 4.4 -- multi-coordinator soak harness

Closes Phase 4 of RFC-21 by adding the soak harness the RFC asks
for: a synthetic-fault-injection test that drives the full
attempt -> evidence -> next-attempt loop across N coordinator
instances and asserts every honest signer arrives at a
byte-identical next-attempt context.

The harness bypasses the receive-loop wiring (which is unit-tested
in pkg/frost/signing under the frost_roast_retry tag) and drives
the Coordinator API directly with synthetic snapshots. The novel
property it exercises is multi-instance agreement: every node's
NextAttempt result must hash-match every other node's, regardless
of which fault-injection scenario was run.

Tests (6 scenarios in multi_coordinator_soak_test.go):

* Clean attempt -- no overflow, no silence -> IncludedSet
  unchanged at next attempt; nothing excluded or parked.
* Overflow exclusion -- 4 observers report 1 overflow each
  against member 3 (sum = OverflowExclusionThreshold) -> member
  3 permanently excluded next attempt.
* Silence parking -- member 3 silent -> member 3 parked at next
  attempt; not permanently excluded.
* Park + reinstate cycle -- N+1 parks member 3 (silent at N);
  N+2 reinstates member 3 (still silent at N+1 by design,
  cannot submit while parked).
* Infeasibility -- threshold = 5 with two silenced members ->
  every node's NextAttempt returns ErrAttemptInfeasible.
* Original signer set preservation -- |Inc| + |Exc| + |Park|
  invariant holds across three consecutive transitions.

Cross-instance agreement is asserted by every soakAttempt invocation:
the helper computes NextAttempt on every node's local Coordinator
instance and refuses to return until every result's hash matches
every other's. A single divergence anywhere causes the test to fail
with a precise hash comparison.

soakSigner produces SHA-256(memberID || payload) signatures; the
matching soakVerifier accepts byte-identical recomputations. No
real crypto needed -- the harness exercises the policy + canonical-
encoding contracts, not key infrastructure.

Verification:

* go test ./pkg/frost/roast/... -- pass
* go test -race ./pkg/frost/roast/... -- pass
* go test -tags 'frost_native frost_tbtc_signer frost_roast_retry'
  ./pkg/frost/... -- pass (5 packages)
* staticcheck -checks '-SA1019' ./pkg/frost/... -- silent
* gofmt -l ./pkg/frost/roast/ -- silent

Stacked on Phase 4.3 (#3974). Closes the Phase 4 surface.

Author: mswilkison