_ed25519_ct_gate.nx

_ed25519_ct_gate.nx -- constant-time gate for SEC-CT-002 (ed25519-scalarmul-constant-time). The FIXED ge_scalar_mul has NO secret-bit branch: each of the 256 bit-iterations runs ge_p3_add UNCONDITIONALLY and masked-selects with ge_p3_cmov. So the number of point additions executed is a CONSTANT 256, independent of the scalar's Hamming weight -- whereas the prior `if bit==1 {ge_p3_add}` ran the add popcount(scalar) times (the variable-time / SPA leak). This gate models BOTH control-flow structures over the EXACT MSB-first 32-byte bit walk ge_scalar_mul uses, on scalars of very different Hamming weight, and asserts: NEW add-count is CONSTANT (256, scalar-independent); OLD add-count VARIES (the NEG control -- proves the gate distinguishes constant-time from variable-time, and that the leak it removes was real). FUNCTIONAL correctness + byte-identity are proven separately and unfakeably by nx_ed25519_signature_test (RFC8032 vectors) + nx_ed25519_scalar_test. Sovereign native: exit 0 = GREEN.