riscv_fp_test.nx

riscv_fp_test.nx -- self-test for rv_emit_fbinop (F-extension). Sanity-check emit_fbinop_mnem and rv_emit_fbinop by hand-building a minimal FADD instruction and checking the emitted asm contains the expected fadd.s mnemonic and register operands. Does NOT attempt to link or execute the fp code path -- the allocator doesn't yet assign f-register homes, so full end-to-end verification waits on the f-reg regalloc commit.