#278 setf-place coverage: ordinals, elt, char, schar, subseq (FIXED)

FIXED 2026-07-11 (588f50e, fd43592). Added setf-expanders: first..tenth, rest, elt (list+vector), char, schar, subseq. Complements the 28 c*r accessors (455b07d/#277). Remaining setf-place gaps need MACRO-level writeback (not function mangling): ldb/mask-field (dpb writeback to place), values (multi-place), fill-pointer. Tracked for future.