w6c: emit length for string-literal .len (fix #14, align cstage to wwstage)

A string literal is TY_UNTYPED_STR, not TY_STR, so `"abc".len` missed
the typed slice/str pseudo-field gate in cgen.c's N_DOT and fell to the
final base-eval fallback, which left AX=.ptr — `.len` returned the
pointer instead of the length. wwstage's cgdot catch-all already did the
BX->AX shuffle, so the two stages diverged (rule-10). Align cstage UP:
the N_DOT fallback emits MOVQ BX,AX for `.len`. `.ptr` is unchanged
(already returned AX); `.cap` deliberately not added (wwstage catch-all
is ptr/len only — mirror exactly).

byte-id was blind here: no bootstrap source uses literal `.len` (lengths
are hardcoded around literals), so the gate never exercised it. New test
801 pins both dimensions (cstage run + cs==ww byte-id) over
len/empty/multibyte/ptr-deref/arg-passthrough rows.

cmd/w6c/cgen.c

@@ -8433,8 +8433,15 @@ cgexpr(Cg *c, Node *n, Local *locals)
 			ins2(c, A_MOVQ, masym(c, n->str), areg(D_AX));
 			break;
 		}
-		/* fall through to base evaluation; result placeholder */
+		/* Non-ident / untyped-str base pseudo-field: e.g. `"abc".len`
+		 * / `"abc".ptr`. A string literal is TY_UNTYPED_STR, not
+		 * TY_STR, so it misses the typed slice/str gate above and
+		 * lands here. cgexpr leaves (AX=ptr, BX=len); `.ptr` keeps AX,
+		 * `.len` shuffles BX→AX. Mirrors wwstage cgdot's catch-all
+		 * (selfhost/cmd/wcc/cgenexpr.ww). #14. */
 		cgexpr(c, n->lhs, locals);
+		if (lenfld)
+			ins2(c, A_MOVQ, areg(D_BX), areg(D_AX));
 	dot_done:
 		break;
 	}