selfhost/cmd/wcc: delete mem.ww (γ-7, Phase 0 close)

mem.ww has 0 callers post-γ-6 — newarena/amalloc/grow/freearena/
roundup all unreferenced after the *arena cascade strip. Drop the
91-line module.

Makefile: remove mem.ww from 5 dep lists (wwdump_ww, w6c_ww,
w6a_ww, w6l_ww, ww_ww); drop `-I selfhost/cmd/wcc` from w6a_ww/
w6l_ww/ww_ww build invocations (wwdump_ww + w6c_ww still need it
for check.ww/cgen*.ww).

test/wcc/990_selfhost.c: drop 6 mem.ww entries from probe_codegen,
probe_dump_diff (×2), probe_resolve, probe_dump_stable, and
probe_cgen_match file lists.

lib/memio/memio.ww: dynamicgrow doc comment reframed as historical
context (collision source is gone, but task #9 keeps the
module-prefixed name conservative against future collisions).

Two dead `import mem;` lines remain in selfhost/cmd/w6a/asm.ww and
selfhost/test/uses.ww; tolerated silently by ww build, swept in
task #8.

main.combined.ww auto-regenerated for w6a/w6c/wwdump.

Verified 132/132 incl. 994_w6c_ww + 995_self_rebuild byte-identity.
Phase 0 closes.

lib/memio/memio.ww

@@ -195,11 +195,11 @@ fn closenoop(s: *io.stream) (void | io.closed) = {
 // small write bursts without a tail of reallocs.
 //
 // `dynamicgrow`, not Hare's bare `grow`: cstage bundles all imported
-// modules into a flat TU and resolves private fns by unqualified
-// name, so two `fn grow` decls (here + selfhost/cmd/wcc/mem.ww's
-// arena `grow`) collide. Module-prefixed name keeps the symmetry
-// with `dynamicwrite`/`dynamicclose` until task #9 (module-aware
-// private-fn scoping in cstage) lands.
+// modules into a flat TU and resolves private fns by unqualified name,
+// so historically two `fn grow` decls (here + the now-deleted bump
+// arena's `grow`) would have collided. Module-prefixed name kept the
+// symmetry with `dynamicwrite`/`dynamicclose`; retained pending task
+// #9 (module-aware private-fn scoping in cstage).
 fn dynamicgrow(m: *state, need: i32) void = {
 	let newcap: i32 = m.cap;
 	if (newcap < 8) { newcap = 8; };