selfhost/cmd/w6a: rename types.ww → opcodes.ww (#11)

The file declares `package w6a;` (not a real `types` module), so
`import types;` was only the bundler file-key. The shadow against
lib/types/ blocked any w6a TU that wanted to pull lib/strings or
lib/bytes (both transitively reach `types.I64_MAX`); the w6a-local
shadow won the source-dir-first resolver, and `types.I64_MAX` came
back undefined → "ordered comparison on non-numeric".

Rename the file to its actual role — opcode + register enums
mirroring 6.out.h — and update the four import lines + Makefile
prereq. No symbol-call sites changed: every constant already
resolves unqualified inside the w6a package.

Unblocks #7 astrndup workaround comments in main.ww / parse.ww;
γ-2 (convert those two sites + delete astrndup export from
selfhost/cmd/wcc/mem.ww) becomes mechanical.

Verified 132/132 incl. 991_w6a_ww.

selfhost/cmd/w6a/obj.ww

@@ -15,7 +15,7 @@ package w6a;
 
 import os;
 import mem;
-import types;
+import opcodes;
 
 // Local wrappers around os.writeall's tagged return — collapse the
 // (i64 | oserror) back to a boolean / int sentinel for the