Fix waveform arithmetic validation and integration

[MooMooCow29]

Summary

This pull request improves the numerical correctness and validation of
waveform arithmetic and integration operations.

Changes

• Validate that channels used in addition, subtraction, and multiplication
  have matching timebases.
• Reject malformed channel data before waveform arithmetic.
• Correctly support multiplication when no custom label is provided.
• Replace constant-step integration with cumulative trapezoidal integration
  using the actual sampling intervals.
• Support nonuniformly sampled waveform data.
• Support valid two-sample integration.
• Reject time vectors that are too short or not strictly increasing.
• Add regression tests for the corrected behaviour.

Motivation

Oscilloscope data may contain nonuniform sample intervals. Using one assumed
sample period can therefore produce inaccurate integrated energy values.

Waveform arithmetic also requires corresponding samples to represent the same
points in time. Explicit timebase validation prevents silent numerical errors
when channels are misaligned.

Testing

Local environment:

• Windows
• Python 3.13.5
• standard-xdrlib installed because xdrlib was removed from Python 3.13

Results:

• Targeted regression tests: 10 passed
• Complete project test suite: 97 passed, 3 warnings
• pycodestyle: passed
• Ruff: All checks passed

The warnings are existing dependency deprecation warnings involving
standard-xdrlib and findiff; they are not failures in this change.

[SevenOfNinePE]

The update is valuable and makes sense. Mainly some updates of unit tests are necessary. Moreover don't hold back on comments.

[SevenOfNinePE]

Do not remove the comment. '# Log missing user input'.
Also if the comment is obviously by code it helps the readability.
In general more comments help to understand the program flow.

[MooMooCow29]

Thanks for reviewing it and for the feedback;

I appreciate it.