Port the deferred sat_subu8/16/32/64 from
ref/hare/math/checked/saturating.ha:196,206,216,226 — clamp to
types.U*_MIN (0) on underflow. Mirrors the existing sat_addu* shape
(typed res forces the sub-word wrap, then the >a underflow test).
Tests: sat_subu* normal+clamp rows plus a direct types min check
(U*_MIN==0, RUNE_MIN=='\0'), wired into checked_test main().
Hojun-Cho
330792b884