fix: validate threshold DKG input parameters

[PROWLERx15]

Fixes #39

run_threshold_keygen did not check t, n, or i up front. Bad
values like t > n would run the full protocol and fail later with
unclear errors deep in ZK proofs or share validation.

Added upfront checks at the start of run_threshold_keygen:

• n < 2 → TooFewParties
• t < 2 → ThresholdTooSmall
• t > n → ThresholdTooLarge
• i >= n → PartyIndexOutOfRange

All return KeygenError via new Bug variants immediately, before any
networking or computation.

Tests (all curves):

• t_greater_than_n t=5, n=3 → error
• t_too_small t=1, n=3 → error
• n_too_small t=2, n=1 → error
• i_out_of_range i=3, n=3 → error