writeToFile fails after a full run if ./data does not exist

### Summary

The latency runner writes its final CSV files under `./data`, but nothing creates that directory for native runs. The documented Docker flow hides this because the bind mount can create the host directory, but running the binary directly can complete the full transaction-sampling run and then fail at the final `os.Create` call.

### Code path

`main.go` writes results with paths such as:

```go
writeToFile(fmt.Sprintf("./data/flashblocks-%s.csv", region), flashblockTimings)
writeToFile(fmt.Sprintf("./data/base-%s.csv", region), baseTimings)
```

`writeToFile` currently calls `os.Create(filename)` directly, so a missing parent directory causes `open ./data/...: no such file or directory`.

### Expected behavior

The runner should create the output directory before writing CSV results so a native run does not discard all collected samples at the very end.

Start in main.go at writeToFile and inspect the existing os.Create calls for the ./data CSV paths. Run the native transaction-sampling flow with no data directory, then verify that the run completes and the expected CSV files are written under ./data.

Written by the indexing model from the issue text.