Restore succeeds with fewer shares than configured threshold

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Description:

In the Python implementation of Shamir SSS, restoration does not enforce the configured threshold.

In pybtc/functions/shamir.py, _interpolation() only requires a minimum of 2 points:

    if k < 2:
        raise Exception("Minimum 2 points required")

combine_mnemonic() in pybtc/functions/bip39_mnemonic.py calls restore_secret() without verifying that the number of shares meets the original threshold.

Additionally, split_secret() does not validate that threshold <= total.

As a result:

• Reconstruction is possible with fewer shares than required (e.g., 2 shares in a 3-of-5 scheme).
• A mnemonic is returned without error.
• Incorrect share configurations (e.g., threshold > total) are not rejected.

Impact:

This may create a false impression of successful recovery and can lead to irreversible fund loss if the reconstructed mnemonic is incorrect.

Request:

Please verify whether, in the 3-of-5 challenge setup:

• Reconstruction with only 2 shares returns a mnemonic without error.
• Adding the correct 3rd share does change the mnemonic (i.e., 2-share output is not presented as a valid recovery).
• threshold > total is properly rejected in your production environment.

Version tested: current pybtc Python implementation.