[P2-E9.1] Two-synchronizer LocalNet topology

**Context.** Nearly every integration test in this milestone needs: gsync (SV, Scan,
validator) + one dedicated synchronizer (own sequencer/mediator, dual-homed operator
participant, one homed validator). Build it once, early — it is the substrate for
P2-E2..E8 acceptance.

**Deliverable.** LocalNet profile with both synchronizers, wired to the bootstrap
scripts (P2-E8.1); fixture support in the Scala integration-test harness; developer doc.

**Acceptance.** CI job boots the topology and runs a smoke test (participant on each sync
can transact); fixture usable from a scalatest suite.

**Depends on.** P2-E8.1 (co-developed).

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Epic: #72 · Plan: [P2-E9-testing.md](https://github.com/ChainSafe/canton-extending-mainnet/blob/main/docs/planning/phase-2-epics/P2-E9-testing.md)