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.

selfhost/cmd/w6a/parse.ww

@@ -338,12 +338,14 @@ fn parseoperand(a: *asm_, p: *u8, offin: u64, n: u64, out: *aoperand) u64 = {
 						out.atype = D_EXTERN;
 						out.asym = dupstr(p + istart, in_);
 						out.offset = symdisp;
-					} else {
-						out.atype = D_INDIR;
-						out.reg = r;
-						out.offset = 0i64;
+						return cur2 + 1u64;
 					};
-					return cur2 + 1u64;
+					// name(REG) with a non-SB register has no
+					// encoding here; the old arm faked D_INDIR
+					// off an UNCHECKED reglookup and silently
+					// discarded the ident and +disp (rule 7).
+					perr(a, "unsupported name(reg) operand");
+					return n;
 				};
 				perr(a, "missing ')'");
 				return n;