lib/ww + wcc + w6c + wwdump: strip *arena cascade (γ-6)

amalloc has 0 callers post-γ-2; the *arena threaded through
newnode/newscope/newtype/prim/typesinit/type{ptr,slice,array,chan,
named}/lexinit/parserinit/joindotted/checkinit/arenau64tos/cgeninit
and the scope.a / tctx.a / lex.a / parser.a / checker.a / cgen.a
fields are vestigial.

Drop `import mem;` from 15 files, remove six struct fields, strip
*arena from 14 signatures, update ~120 call sites across lib/ww +
wcc + w6c + wwdump. selfhost/test/sym_link.ww fixture drops the
newarena/freearena probe; still exits 42 on scopedefine/scopelookup.
Both main.combined.ww auto-regenerated.

Comments retidied: typ.ww "once per arena" → "once per program";
parse.ww drops "arena-build" qualifier on joindotted; sym.ww drops
mem-sibling-imports rationale.

Verified 132/132 incl. 994_w6c_ww + 995_self_rebuild byte-identity
(the primary symmetric-stages gate).

selfhost/cmd/wcc/cgenutil.ww

@@ -15,7 +15,6 @@
 package wcc;
 
 import os;
-import mem;
 import ast;
 import tok;
 import typ;
@@ -30,7 +29,7 @@ import strconv;
 // (caller side) both advertise their effective type as []ELEM —
 // every isslicetype / nodeisslice check then succeeds naturally.
 fn slicewrap(c: *cgen, elem: *node) *node = {
-	let s: *node = newnode(c.a, nkind.N_TSLICE, "", 0, 0);
+	let s: *node = newnode(nkind.N_TSLICE, "", 0, 0);
 	s.lhs = elem;
 	return s;
 };