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.

selfhost/cmd/wcc/mem.ww

@@ -1,91 +0,0 @@
-// selfhost/cmd/wcc/mem.ww — port of cmd/wcc/mem.c.
-//
-// Bump arena allocator. Backed by the runtime page allocator
-// (rt_malloc / rt_free), no libc. Each chunk is mmap'd; when the
-// current chunk runs out we link a fresh one. Freeing the arena
-// unmaps the chain.
-//
-// Memory handed out is 16-byte aligned. The C version under
-// cmd/wcc/ is retained until the three-stage bootstrap diffs clean.
-
-package wcc;
-
-import os;
-import rt;
-
-def ALIGN: u64       = 16u64;
-def INIT_CHUNK: u64  = 65536u64;
-def MAX_CHUNK: u64   = 4194304u64;
-def ARENA_SZ: u64    = 48u64;	// sizeof(arena), kept in sync below
-
-type arena = struct {
-	buf:   *u8,
-	off:   u64,
-	cap:   u64,
-	next:  *arena,
-	total: u64,
-};
-
-fn roundup(n: u64, a: u64) u64 = {
-	return (n + a - 1u64) & ~(a - 1u64);
-};
-
-export fn newarena() *arena = {
-	let a: *arena = rt.malloc(ARENA_SZ): *arena;
-	a.buf = rt.malloc(INIT_CHUNK): *u8;
-	a.off = 0u64;
-	a.cap = INIT_CHUNK;
-	a.next = nil;
-	a.total = 0u64;
-	return a;
-};
-
-// Grow: link a fresh chunk in front of the head. We push the old
-// chunk into `next` so the head always describes the current bump
-// region. Chunk size doubles up to MAX_CHUNK.
-fn grow(a: *arena, need: u64) bool = {
-	let want: u64 = a.cap * 2u64;
-	if (want < need) { want = need; };
-	if (want > MAX_CHUNK) { want = MAX_CHUNK; };
-	if (want < need) { return false; };	// single allocation too big
-
-	let old: *arena = rt.malloc(ARENA_SZ): *arena;
-	old.buf = a.buf;
-	old.off = a.off;
-	old.cap = a.cap;
-	old.next = a.next;
-	old.total = 0u64;
-
-	a.buf = rt.malloc(want): *u8;
-	a.off = 0u64;
-	a.cap = want;
-	a.next = old;
-	return true;
-};
-
-export fn amalloc(a: *arena, n: u64) *void = {
-	let need: u64 = roundup(n, ALIGN);
-	if (need > a.cap - a.off) {
-		if (!grow(a, need)) { return nil; };
-	};
-	let p: *u8 = a.buf + a.off;
-	a.off += need;
-	a.total += need;
-	// Zero the region. Plan 9 amalloc zeroes; we mirror that here so
-	// the checker can assume freshly allocated nodes start at 0.
-	let i: u64 = 0u64;
-	for (i < need) {
-		p[i] = 0u8;
-		i += 1u64;
-	};
-	return p: *void;
-};
-
-export fn freearena(a: *arena) void = {
-	for (a != nil) {
-		let next: *arena = a.next;
-		os.free(a.buf: *void, a.cap);
-		os.free(a: *void, ARENA_SZ);
-		a = next;
-	};
-};