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/cgenexpr.ww

@@ -15,7 +15,6 @@
 package wcc;
 
 import os;
-import mem;
 import ast;
 import tok;
 import typ;
@@ -3172,7 +3171,7 @@ fn cgcall(c: *cgen, n: *node) void = {
 				emitline("\tMOVQ\tAX, ");
 				emitoff((soff + 16): i64);
 				emitline("(BP)\n");
-				let sn: *node = newnode(c.a, nkind.N_IDENT,
+				let sn: *node = newnode(nkind.N_IDENT,
 				    "", 0, 0);
 				sn.str = sname;
 				if (prevarg == nil) { n.list = sn; }