Architecture allocation

TASK: Function-to-ECU allocation.

Assign every function to exactly one ECU so that ALL constraints hold:
- the sum of cpu of the functions on an ECU <= its cpuCap, and the sum of pins <= its pinCap;
- a function with a zone must go on an ECU in that zone (zone null = any ECU);
- each pair in `segregate` must be on different ECUs.
Among valid allocations, MINIMISE crossRate = the sum of `rate` over signals whose two functions are on different ECUs.

ECUs:
[
  {"id":"E1","cpuCap":35,"pinCap":11,"zone":"FRONT"},
  {"id":"E2","cpuCap":42,"pinCap":16,"zone":"CABIN"},
  {"id":"E3","cpuCap":74,"pinCap":41,"zone":"REAR"}
]

Functions:
[
  {"id":"F1","cpu":23,"pins":11,"zone":null},
  {"id":"F2","cpu":12,"pins":9,"zone":"REAR"},
  {"id":"F3","cpu":5,"pins":7,"zone":null},
  {"id":"F4","cpu":22,"pins":6,"zone":null},
  {"id":"F5","cpu":10,"pins":7,"zone":"REAR"},
  {"id":"F6","cpu":28,"pins":12,"zone":null}
]

segregate: []

Signals:
[
  {"a":"F2","b":"F1","rate":340},
  {"a":"F1","b":"F5","rate":480},
  {"a":"F4","b":"F3","rate":190},
  {"a":"F2","b":"F5","rate":110},
  {"a":"F6","b":"F3","rate":370},
  {"a":"F1","b":"F4","rate":230},
  {"a":"F1","b":"F3","rate":360},
  {"a":"F1","b":"F6","rate":500}
]

Output format (JSON, exactly these keys):
```json
{
 "allocation": [
  {
   "function": "F1",
   "ecu": "E2"
  }
 ],
 "crossRate": 340
}
```