test: add comprehensive Signal facade coverage tests

Added tests for all previously untested Signal methods:
- FFT methods (fft, ifft, rfft, irfft, fft2, ifft2, stft, istft)
- DCT methods (dct with ortho/normal, idct)
- Spectral analysis (welch, periodogram, spectrogram)
- Convolution/correlation (1D and 2D, all modes)
- Filtering (savgol_filter, medfilt)
- Peak analysis (peakProminences, peakWidths)
- FIR design (firwin variants)
- Utility methods (sosfilt, padSignal)

Coverage improvements:
- Signal.java facade: 76% -> 96% (+20%)
- Overall instruction: 95% -> 95% (maintained)
- Overall line: 94.4% -> 95.1% (+0.7%)
- Method coverage: ~92% -> 95.1% (+3%)

All tests pass with golden master validation.

Bump to v3.1.7

Bump to v3.1.6

Fix Maven Central credentials: Use ORG_GRADLE_PROJECT_ prefix

chore: bumped version to 3.1.4 and updated javadocs

bump to v3.1.3

Bump to v3.1.2

Adding figures within javadoc

bump to v3.1.0

bump version v3.0.2