`let s: str = *h` (a str/slice loaded by value through *str / *[]T)
fell through the N_UN deref arm to the scalar load, emitting a single
MOVQ that read only the 8B .ptr and left .len/.cap from stale registers,
so len(*p) returned garbage. Both stages emitted byte-identical wrong
code, so the self-compile byte-id gate was blind to it. Add a str/slice
arm that loads the full {ptr,len,cap} via cgslicehdr when the chased
pointee is TY_STR/TY_SLICE.
Surfaced by the codegen miscompile hunt (finding C1b). Pinned by
test/lang/deref_hdr_test.ww, which interposes a different-sized decoy
header so the test reddens when the arm is reverted.
Hojun-Cho
c67f362bbd