Add missed-ack-escalation scenario: 3 dropped prepare_acks force retr…

Description

The pitch.

## What
New scenario testing consensus recovery when acknowledgments are dropped.

- 1 proposer + 5 acceptors
- 3 of 5 prepare_acks dropped (quorum = 3, so 2 survive → below quorum)
- Proposer retries missing acceptors, reaches quorum on a later round

## Properties (all pass)
- quorum_commit: commit followed 3 distinct eligible acks
- agreement: all 5 acceptors committed the same value
- fault_recovered: drops → proposal_retry → recovery
- ledger_conserved

## Run
```bash
pip install nandatown
nandatown run missed-ack-escalation   
```
## Evidence
runs/sim-88819bb84033