Fix ADC parameter calculation overflow errors

[briangow]

As discussed in #515, an overflow can occur when calculating ADC parameters from limited-precision physical signal data. The root cause of this overflow is precision loss. When computing min/max values during parameter calculation, insufficient floating-point precision at boundaries (e.g. int32 boundaries) leads to gain/baseline values that cause digital samples to overflow by exactly 1 value.

The fix ensures float64 precision is used throughout ADC parameter calculation and conversion:

• In calc_adc_params(): Convert physical signals to float64 before computing min/max
• In adc_inplace(): Convert to float64 before performing gain/baseline arithmetic

Users can now safely provide lower-precision physical data (e.g., float32) without validation errors.

This PR includes a test for the overflow which uses the data supplied by @Ivorforce in #515. The test checks that:

• float32 input data can be written with fmt=32 without overflow
• digital values stay within the valid range
• the physical signal round-trips accurately through write/read