Add RFAmplifier and RFMixer blocks

[milanofthe]

Summary

• RFAmplifier: Algebraic block with linear voltage gain and optional soft saturation (tanh compression). Ports: rf_in → rf_out.
• RFMixer: Ideal frequency converter via time-domain multiplication. Ports: rf, lo → if_out.
• Both implemented as Function blocks following existing codebase patterns.
• 21 new tests (11 amplifier + 10 mixer), all 32 tests passing.