w6a: DATAW directive for writable .data section

First step toward top-level mutable `let`. Adds a sibling directive to
DATA whose bytes land in a separate writable .data PROGBITS section
(SHF_ALLOC|SHF_WRITE, STT_OBJECT) instead of .text. The section is
emitted only when DATAW was used, so inputs without it produce a
byte-identical .o — tests 991 (selfhost .o diff) and 995 (self-rebuild)
keep passing unchanged.

w6l still treats data-resident syms as undefined; that's the next step.

cmd/w6a/parse.c

@@ -120,6 +120,7 @@ opcode_lookup(const char *m)
 		{ "SYSCALL", A_SYSCALL },
 		{ "TEXT", A_TEXT },
 		{ "DATA", A_DATA },
+		{ "DATAW", A_DATAW },
 		{ NULL, 0 }
 	};
 	for (int i = 0; tab[i].m; i++)
@@ -324,8 +325,9 @@ a_parse(Asm *a)
 				prg->from.type = D_CONST;
 				prg->from.offset = a_parsenum(m, NULL);
 			}
-		} else if (op == A_DATA) {
-			/* DATA name(SB),"escaped bytes" */
+		} else if (op == A_DATA || op == A_DATAW) {
+			/* DATA  name(SB),"escaped bytes" — read-only in .text
+			 * DATAW name(SB),"escaped bytes" — writable in .data */
 			char nbuf[128] = {0};
 			int nn = 0;
 			while (*m && *m != '(' && nn < 127) nbuf[nn++] = *m++;