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).

lib/ww/lex/lex.ww

@@ -17,7 +17,6 @@ package lex;
 // callers `import lex;` (which dir-enums lib/ww/lex/).
 import os;
 import ascii;
-import mem;
 import strings;
 
 // isidstart / isidpart — identifier classification. Lexer-local
@@ -55,18 +54,16 @@ type lex = struct {
 	lpos:   u64,
 	line:   i32,
 	col:    i32,
-	a:      *arena,
 	errs:   i32,
 };
 
-export fn lexinit(l: *lex, a: *arena, file: str, src: *u8, len: u64) void = {
+export fn lexinit(l: *lex, file: str, src: *u8, len: u64) void = {
 	l.file = file;
 	l.src = src;
 	l.srclen = len;
 	l.lpos = 0u64;
 	l.line = 1;
 	l.col = 1;
-	l.a = a;
 	l.errs = 0;
 };