M7-T06 Promote replicated simulation scenarios into executable tests

[skel84]

Summary

Promote the replicated simulation scenarios from the design docs into executable regression tests.

Why

The M6 simulation plan only becomes meaningful once partition, primary-crash, and rejoin cases run against the real replicated harness and can reproduce failures from seed plus transcript.

Scope

• encode the required partition scenarios from docs/testing.md
• encode the primary-crash retry-boundary scenarios from docs/testing.md
• encode the rejoin and recovery scenarios from docs/testing.md
• keep transcripts replayable and shrinkable enough for regression debugging

Acceptance Criteria

• partition, primary-crash, and rejoin scenarios execute in the real replicated harness
• the same seed and starting state reproduce the same replicated transcript
• promoted tests check the invariants documented in docs/testing.md
• failing schedules can be replayed directly from recorded transcript data

Validation

• ./scripts/preflight.sh

Planning Reference

• milestone: M7 Replicated Core Prototype
• task id: M7-T06
• blocked by: M7-T05