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/cgen.ww

@@ -25,7 +25,6 @@
 package wcc;
 
 import os;
-import mem;
 import ast;
 import tok;
 import typ;
@@ -68,9 +67,9 @@ fn collectaliases(c: *cgen, file: *node) void = {
 	// same-module / any-match passes in aliaslookup then let a local
 	// `type nomem = !void;` shadow this fallback within its module.
 	let empty: str;
-	let tnvoid: *node = newnode(c.a, nkind.N_TNAME, empty, 0, 0);
+	let tnvoid: *node = newnode(nkind.N_TNAME, empty, 0, 0);
 	tnvoid.str = "void";
-	let bang: *node = newnode(c.a, nkind.N_TBANG, empty, 0, 0);
+	let bang: *node = newnode(nkind.N_TBANG, empty, 0, 0);
 	bang.lhs = tnvoid;
 	let nomemal: *aliasent = alloc(aliasent{aname="nomem", amod=empty, target=bang, aanext=nil})!;
 	c.aliases = nomemal;
@@ -435,7 +434,6 @@ def LOOP_MAX: i32 = 16;
 def DEFER_MAX: i32 = 16;
 
 type cgen = struct {
-	a:        *arena,
 	locals:   *local,
 	// atlocals — persistent registry of `@`-prefix scratch slots
 	// for the current fn. cgblock save/restores c.locals to scope
@@ -522,8 +520,7 @@ type letvar = struct {
 	lvnext: *letvar,
 };
 
-fn cgeninit(c: *cgen, a: *arena) void = {
-	c.a = a;
+fn cgeninit(c: *cgen) void = {
 	c.locals = nil;
 	c.atlocals = nil;
 	c.frame = 0;