selfhost/cmd/w6a: strip *arena cascade (γ-3)

amalloc has 0 callers post-γ-2; the *arena threaded through w6a's
init/dupstr/bufinit and the asm_.a / buf.a fields are vestigial.

Drop `import mem;` from opcodes/parse/obj/main, remove asm_.a and
buf.a struct fields, strip *arena from init/dupstr/bufinit
signatures, update 12 call sites. main.combined.ww auto-regenerated.

Comment at main.ww:36 retidied "argv/arena" → "argv-style" to match
post-strip reality.

Verified 132/132 incl. 991_w6a_ww + 995_self_rebuild byte-identity.

selfhost/cmd/w6a/opcodes.ww

@@ -3,8 +3,6 @@
 
 package w6a;
 
-import mem;
-
 // ---- registers + operand kinds (from 6.out.h) -------------------------
 // These must stay numerically aligned with the C enum so that ww-cgen
 // output (which reads them via `D_AX(SB)` etc.) lands on the same
@@ -190,7 +188,6 @@ type afixup = struct {
 };
 
 type asm_ = struct {
-	a:       *arena,
 	file:    str,
 	src:     *u8,
 	srclen:  u64,