#2872 bench.sh crashes with NameError when it starts the server itself (regression from #2330)

Description

### What happened

`./bench.sh` (with no `--url`) crashes before it starts the model:

```
  File "tools/strata_bench.py", line 990, in main
    say(f"Starting {cfg_path.name} on port {port} (loading takes 1-3 minutes; the PC may be slow meanwhile) ...")
NameError: name 'port' is not defined
```

Commit 69fe4055 (#2330) replaced

```python
port = a.port or int(cfg.get("port", 8080))
base = f"http://127.0.0.1:{port}"
```

with `base = server_base(cfg.get("host"), a.port or int(cfg.get("port", 8080)))`, but `port` is still used at lines 987, 990 and 991 (`start_server(cfg_path, port, ...)`). Every run where bench.sh starts the server hits this. Running with `--url` against an already running server works, because it skips that branch.

To reproduce: a set-up install with no server running, then `./bench.sh`.

Suggested fix: keep the variable.

```python
port = a.port or int(cfg.get("port", 8080))
base = server_base(cfg.get("host"), port)
```

Workaround until then: start the model with `./run-<model>.sh`, then `./bench.sh --url http://127.0.0.1:<port>`.

### Strata version, GPU, OS

Engine 0.1.45 (main at 477ac13c, still present on main today). RTX 3090, Linux (Ubuntu 24.04 base), engine compiled from source by setup.