Estimate SNR of Signal

Estimate the signal-to-noise ratio of a measured sine wave by comparing the power in the fundamental bin against the power in everything else.

Constraints

• Function: snr_db = estimate_snr(x, fs).
• Apply a window before the FFT to limit spectral leakage.
• Exclude the DC bin and the fundamental's immediate neighbours from the noise.
• Return the result in decibels.
• A constant signal must return -Inf or an error, not a NaN.
• Fewer than 8 samples must error.