featreq: Add `MIXED` mode to combine deployment and call transactions

## Description

Supernova only runs one transaction type at a time. Production networks handle mixed workloads (deployments + calls), so current benchmarks may not reflect real-world performance.

## Proposal

Add a `MIXED` mode with configurable transaction ratios:

```bash
-mode MIXED -mix-ratio "REALM_CALL:70,REALM_DEPLOYMENT:20,PACKAGE_DEPLOYMENT:10"
```

(this is an example format)