test(tbtc): multi-signer simulated integration test for reservation coordination

[piotr-roslaniec]

Summary

Implements implementation-plan.md Milestone 3's "multi-signer simulated
integration test" item - the last piece of the full M1 keep-core-readiness
implementation plan (M0 is an external release-coordination gate, not a code
task; M1's four rows and M2's test-coverage backfill are covered by
#4276,
#4277,
#4278, and a
separate M2 follow-up PR).

Stacked on m1/reservation-coordination-checklist
(#4278), tip
c8eb5419f.

Scope note (per explicit decision this session): Milestone 3 has two
items - this test, and a "testnet round with a forced liveness/stranding
drill" (~2 weeks, needs a live testnet deployment and real multi-operator
wall-clock timing). Only the former is code; the latter is tracked as an
agent-not-actionable item in the plan doc, unchanged by this PR.

Change

Scales TestCoordinationExecutor_Coordinate's existing 3-operator harness -
deterministic keypairs, real per-operator localChain fakes, a real shared
netlocal.BroadcastChannel, one goroutine per operator running
coordinationExecutor.coordinate concurrently - to ReservationAnchorProposal
and ReservationReanchorProposal:

• TestCoordinationExecutor_Coordinate_ReservationAnchor
• TestCoordinationExecutor_Coordinate_ReservationReanchor

This exercises the real leader/follower coordination round-trip (checklist
generation -> leader election -> broadcast -> follower validation ->
convergence) that no mocked pkg/tbtcpg unit test can cover, since those
call task.Run(request) directly and never go through
coordinationExecutor.coordinate. It also exercises #4277's protobuf
marshaling of both proposal types over a real wire round-trip, since every
follower unmarshals the leader's broadcast coordinationMessage.

Depends on #4278 (this branch's parent): before that fix,
ActionReservationAnchor/ActionReservationReanchor never appeared in
getActionsChecklist's output, so every operator's checklist search in
these tests fell through to NoopProposal and failed. Verified directly:
temporarily reverted #4278's checklist change, re-ran both new tests (both
failed with the expected NoopProposal mismatch), then restored it.

A bug found in this test's own harness

Both new tests initially shared one netlocal broadcast channel name.
getBroadcastChannel's registry is keyed by name and never releases old
channels, so under -race (which changed goroutine/channel-delivery timing
enough to surface it in ~every run), the reanchor test's follower sometimes
received a stale broadcast left over from the anchor test's leader - a
cross-test data race in the test harness itself, not in the production code
under test. Fixed by giving each test its own channel name; re-verified
stable across 10 repeated -race runs plus the full non-race and race
suites.

Testing

• go test ./pkg/tbtc/...: 365/365 pass.
• go test -race ./pkg/tbtc/...: clean, no data races, including
  -count=10 targeted at just the two new tests.
• go build ./... && go test ./...: full repo, 49 packages, zero FAIL.
• gofmt -l / go vet: clean.

Not in this PR

• The testnet-round liveness/stranding drill (Milestone 3's other item) -
  operational, not code; tracked separately.
• Milestone 2's test-coverage backfill - separate follow-up PR.

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