[P2-E9] Testing & acceptance

The integration substrate and the milestone's proof: a two-synchronizer LocalNet, the
end-to-end acceptance scenario that is Phase 2's exit criterion, and CI hygiene so the
suites gate every change.

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Milestone: Phase 2 · Plan: [P2-E9-testing.md](https://github.com/ChainSafe/canton-extending-mainnet/blob/main/docs/planning/phase-2-epics/P2-E9-testing.md) · Leaves filed as sub-issues.

Start with docs/planning/phase-2-epics/P2-E9-testing.md to identify the leaves and their acceptance criteria. The work described here spans a two-synchronizer LocalNet, a Phase 2 end-to-end acceptance scenario, and CI hygiene; it is done when those suites gate every change and the milestone exit criterion passes.

Written by the indexing model from the issue text.