fix(sensordash): preserve negative peaks in downsampling

[taigrr]

Summary

• move pure sensordash rendering helpers into an internal package with unit tests
• preserve the largest absolute value when downsampling waveform buckets so negative spikes are not flattened away
• keep ANSI width handling covered outside the Darwin-only dashboard entrypoint

Testing

• go test ./...
• /home/tai/go/bin/staticcheck ./...