w6a: bare negative constant parses honestly; name(reg) rejects

Two parse.c operand defects. A bare `-8` (no $) double-negated to
+8: a_parsenum lets strtoll consume the sign, and the caller's
`off * sign` re-applied it (the D_INDIR arm never multiplied, which
is why -8(BP) always worked); the wwstage twin was already correct,
so this was a latent cs!=ww on hand-written source. The name(REG)
non-SB arm faked a D_INDIR off an UNCHECKED reg_lookup and silently
discarded the parsed ident and +disp -- both stages; the shape has
no encoding and now rejects loud.

cmd/w6a/parse.c

@@ -197,8 +197,11 @@ parse_operand(Asm *a, const char *s, Aoperand *out)
 			out->offset = off;
 			return 0;
 		}
+		/* a_parsenum consumed the sign itself (strtoll); the local
+		 * `sign` exists only so isdigit() sees past the '-'.
+		 * Multiplying re-applied it: bare `-8` parsed as +8. */
 		out->type = D_CONST;
-		out->offset = off * sign;
+		out->offset = off;
 		return 0;
 	}
 
@@ -233,11 +236,12 @@ parse_operand(Asm *a, const char *s, Aoperand *out)
 				out->offset = sym_disp;
 				return 0;
 			}
-			out->type = D_INDIR;
-			out->reg = r;
-			out->offset = 0;
-			/* unusual case: name(REG) with named offset; not used */
-			return 0;
+			/* name(REG) with a non-SB register has no encoding
+			 * here; the old arm faked D_INDIR off an UNCHECKED
+			 * reg_lookup and silently discarded the ident and
+			 * +disp (rule 7). */
+			err(a, "unsupported name(reg) operand");
+			return -1;
 		}
 
 		int r = reg_lookup(buf);