Feature request: Scenario snapshot/restore to skip redeployment

[vbuilder69420]

Problem

Every benchmark run deploys contracts + runs setup transactions (~40s for a 6-contract DeFi scenario). For A/B performance testing where we run 4-8 iterations of the same scenario, this is pure overhead — the contracts are identical each time.

Proposed Solution

contender snapshot --scenario sharedEconomy.toml --output snapshot.json — save deployed contract addresses and DB state after setup.

contender spam --from-snapshot snapshot.json — skip deployment entirely, start spamming against known addresses.

Alternatively, a --reuse-db flag that detects contracts are already deployed at the expected addresses and skips setup.

Impact

Saves ~40s per benchmark run. For an 8-run A/B experiment, that's 5+ minutes of pure setup overhead eliminated. Also removes all setup-related variance from measurements.

[zeroXbrock]

This need to skip deployments/setup in a spam run only applies to builtin scenarios, which can already be done with the --skip-setup flag.
But for file-based scenarios (what's being used in the issue description), setup doesn't happen when calling spam. "Snapshots" as described in this issue are redundant; the spammer already finds these addresses in the DB in the same way that the snapshot does.

What does a "benchmark run" entail? I can't imagine 40s being saved by replacing DB lookups with a file read + JSON parse.