Mirrors cgen.c §5130-5223 / cgfnparams: reg-spill, tagged-partial-fit, pure-stack. Pure-stack does not bump cursor (cstage semantics). Fixes 16B over-allocation on 7+ arg functions; bootstrap stays byte-identical. Slice/str at reg/stack straddle is deferred to task #11 (cgfnparams doesn't stitch them either); pre-scan stays symmetric until then.