w6c: 'as' on a module-global tagged ident loads the full box

cstage's N_TYPEASSERT assumed cgexpr filled the registers and spilled
an uninitialized payload (cs=0 for any stored value); mirror the
landed wwstage emission (tag/payload/cap from g(SB)). Flips the
residual row to cs==ww==correct. Task #46.

cmd/w6c/cgen.c

@@ -10716,7 +10716,22 @@ cgexpr(Cg *c, Node *n, Local *locals)
 		if (sl_off == 0) {
 			sl_off = localoff(c, &locals, "@asrt_spill",
 			    slot_size, cg_frame);
-			if (cg_tagged_memread(s)) {
+			if (s && s->kind == N_IDENT && let_islet(s->str)) {
+				/* #46 (#263): a module-global tagged ident —
+				 * cgexpr loads only the tag word (MOVQ g(SB),AX),
+				 * so the DX/CX spill below would write
+				 * uninitialized payload. Copy the whole box from
+				 * g(SB)+0/+8[/+16] into the spill so the
+				 * tag-check + payload load index off memory like
+				 * a local. ww half landed F8-c3. */
+				ins2(c, A_LEAQ, masym(c, s->str), areg(D_AX));
+				for (int k = 0; k < slot_size; k += 8) {
+					ins2(c, A_MOVQ, amem(D_AX, k),
+					    areg(D_DX));
+					ins2(c, A_MOVQ, areg(D_DX),
+					    amem(D_BP, sl_off + k));
+				}
+			} else if (cg_tagged_memread(s)) {
 				/* #37: >32B box read — ADDRESS in AX; copy
 				 * the whole box from memory. */
 				cgexpr(c, s, locals);