Probe-first find for the path c2 appendlit (buf.buf[lo..hi]=bs): a
slice-copy-assign into a struct-field array sub-range emitted ZERO code —
silent NO-OP, both stages, both-wrong-identical (#263), so runtime is the
only net. N_ASSIGN gains an N_SLICE-LHS arm (cgen.c + cgenexpr.ww
slicebaseesz twin) reusing the N_SLICE-read base/esz cascade and copying
(hi-lo)*esz bytes from rhs.ptr via a runtime loop (len is runtime; no
REP/MOVSB). esz routed through the type table (rule 13; [N]u8->1). Hare
len(bs)==hi-lo assert deferred to #149.
