nx_bigint_modexp.nx

nx_bigint_modexp.nx -- modular reduction + modular exponentiation on nx_bigint (the missing keystone). nx_bigint has add/sub/mul/shift/cmp but NO reduction, so it couldn't do a^e mod m -- the primitive under Diffie-Hellman (hence BitTorrent MSE/PE anti-throttle encryption), RSA, and any modular crypto. This adds: bi_mod -- rem = a mod m via bit-by-bit long division (shift-subtract), using only bi_shl1/cmp/sub. bi_modexp -- result = base^exp mod m via square-and-multiply, reducing after every multiply. Little-endian u32 limbs, same representation as nx_bigint. Caller owns all limb buffers (BI3). Correctness first (not constant-time yet -- a DH/RSA hardening note; fine for the KAT + the anti-throttle handshake bring-up). nx_bigint_modexp -- no-arg = the GATE (classic + Fermat + multi-limb-reduction vectors) license_tier: ORIGINAL module: nishi-core.crypto.bigint_modexp depends: nishi-core.bigint