selfhost/cmd/w6a: align parsenum to strtoll(base 0) semantics (#62)

w6a's parsenum diverged from the C twin's strtoll(s,end,0)
(cmd/w6a/lex.c:30) on three hand-written-asm edge shapes (all
gate-blind — w6c emits the canonical $5/$8/-8(BP), never these):
  (a) `$ 5`  — leading whitespace: strtoll skips it (->5); ww had no
              skip and silently encoded imm 0.
  (b) `$08`  — strtoll base-0 reads a leading 0 as octal, stops at '8'
              (->0); ww parsed it as decimal 8.
  (c) `-(BP)` — strtoll/cstage require a digit after the sign, so a bare
              `-(` is unrecognised operand; ww silently took it as 0(BP).
Add the whitespace skip + octal base-0 detection to parsenum, and the
digit-after-sign guard to the operand scanner — both assemblers now
agree byte-for-byte (a/b) and both reject (c).

Not a Hare item (w6a is ww's plan9-lineage assembler); reference is the
C strtoll twin. w6a embeds into its own combined.ww snapshot; regen'd.
530_w6a_parsenum pins the byte-identity + both-reject matrix.

selfhost/cmd/w6a/parse.ww

@@ -273,7 +273,17 @@ fn parseoperand(a: *asm_, p: *u8, offin: u64, n: u64, out: *aoperand) u64 = {
 	let cur: u64 = off;
 	let isnum: bool = false;
 	if (cur < n) {
-		if (p[cur] == '-') { isnum = true; }
+		// cstage parse.c:189-190 strips '-' then requires isdigit(*p);
+		// a bare `-(BP)` is NOT a number operand and falls through to a
+		// loud reject. ww treated '-' alone as a number → silent 0(BP).
+		// Require a following digit to match (#62).
+		if (p[cur] == '-') {
+			if (cur + 1u64 < n) {
+				if (p[cur + 1u64] >= '0') { if (p[cur + 1u64] <= '9') {
+					isnum = true;
+				}; };
+			};
+		}
 		else { if (p[cur] >= '0') { if (p[cur] <= '9') { isnum = true; }; }; };
 	};
 	if (isnum) {