NilPy: raise def/method parameter limit past 4

Part of [[feature-nilpy-corpus-uforth]] milestone 1. uforth dataclasses need ctors with 5-7 params (Word has 6 fields + self); the v1 NilPy prologue spills only the first 4 integer registers (rdi/rsi/rdx/rcx) and errors — worse, PyParseMethod's spill case has no else and SILENTLY drops the spill for params 5+ (garbage reads, no diagnostic).

Fix: spill r8/r9 (SysV args 5-6) and copy stack-passed params 7+ from [rbp+16+8*(i-6)] into their frame slots, both in PyParseDef and PyParseMethod. Limit becomes MAX_PROC_PARAMS-bounded.

Gate

test-nilpy green (+ regression test_nilpy_many_params.npy with 8-param def), self-host byte-identical, testmgr quick.