go-ipld-prime's DAG-CBOR and DAG-JSON decoders have unbounded recursion depth

The DAG-CBOR and DAG-JSON decoders recurse on each nested map or list without a depth limit. A payload containing deeply nested collections causes the decoder to recurse once per level, growing the goroutine stack until the Go runtime terminates the process with a fatal stack overflow (distinct from a recoverable panic).

For DAG-CBOR, a payload of approximately 2 MB, consisting of repeated 0x81 (array-of-1) bytes followed by a terminator, produces around 2 million recursion frames and reliably exhausts Go's default 1 GB goroutine stack. The existing allocation budget does not prevent this: each nested collection header costs only a handful of budget units, so the stack is exhausted before the budget is. DAG-JSON has equivalent exposure via [[[...]]]-style payloads; it has no budget system and is therefore unprotected against recursion depth as well.

Schema-free decoding (using basicnode.Prototype.Any) allows arbitrary nesting depth. Schema-bound decoding bounds nesting only when the schema itself is non-recursive and contains no fields typed as Any; schemas with recursive type references or any Any-typed fields permit unconstrained nesting at those points.

The fix adds a configurable MaxDepth option to both decoders, defaulting to 1024 nested levels. The decoder returns ErrDecodeDepthExceeded when a payload nests beyond the limit. Well-formed IPLD data rarely approaches this depth in practice; the default is generous for legitimate use while preventing stack exhaustion.