nx_riscv_loop.nx

nx_riscv_loop.nx -- SOVEREIGN RISC-V backend, RUNG 3: CONTROL FLOW. Now uses the GENERAL nx_riscv_lib for the shared toolchain (encoders, lower with control flow, build_header, emulator) -- only the loop-IR + tests are local. Lowers/runs: acc=0; i=1; L0: acc += i; i += 1; if i <= N goto L0; return acc (triangular sum 1..N) T1 the lowered loop contains a real back-edge branch (0x63) and ends in ecall. T2 run N=5 -> exit 15. T3 (teeth) re-lower N=10 -> exit 55. expect_exit: 0 Sovereign: nx_riscv_lib (+ nx_syscalls).