Reading gp.f through a module-global pointer miscompiled in BOTH stages,
differently: cstage classified gp as a local at boff 0 and derefed BP
(MOVQ (BP),BX), wwstage collapsed gp.f to an undefined global symbol f
(MOVQ f(SB)). Both now load the pointer value from the global's data slot
before the field offset, converging on MOVQ gp(SB),BX; MOVQ off(BX),AX.
cstage mirrors the #6 store decline; wwstage gains a global-ptr arm and
shares a cgptrfieldload helper with the local arm.
Fused, not split: the two stages must emit byte-identical asm, so a
one-stage commit would fail the byte-id gate. Sibling byte-divergences
filed: #16 (chained-spine gp.x.y), #17 (>32B tagged word-order).
Test: table-driven 689_globptr_field_read_run (24 rows, runtime + byte-id).