The standardResolver's lookup4/lookup6 methods recursively follow CNAME
chains without any depth limit or cycle detection. A malicious or
misconfigured DNS server returning a self-referential CNAME record
(e.g. loop.example. CNAME loop.example.) causes infinite recursion,
leading to stack overflow and crash.
Add a max depth limit of 16 and a visited-set to detect cycles. The
original lookup4/lookup6 entry points are preserved; they delegate to
lookup4WithCNAMEDepth/lookup6WithCNAMEDepth which track depth and seen
hosts across recursive calls.
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