overlay/checksum: test each arch implementation directly

The correctness sweeps only exercised the public Checksum dispatcher, so
wherever it resolved to the gvisor fallback (non-AVX2 amd64, fallback
architectures) the suite compared gvisor against itself and the AVX2
assembly went untested -- silently green. Per-arch export_test.go files
now enumerate the hand-written implementations and every sweep runs
against the dispatcher plus each of them, skipping with an explicit
message when the running CPU can't execute one.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

overlay/checksum/export_amd64_test.go

@@ -0,0 +1,11 @@
+package checksum
+
+// archImpls exposes every hand-written implementation on this architecture
+// so the tests exercise them directly, independent of what the public
+// Checksum dispatches to on the running CPU. Without this, running the
+// suite on a non-AVX2 machine compared gvisor against itself and left the
+// assembly untested — silently. available=false makes the test skip loudly
+// instead.
+var archImpls = []archImpl{
+	{name: "avx2", fn: checksumAVX2, available: hasAVX2},
+}