Parse a full serialized block (header + varint tx count + transactions).
This is how an L3 verifier obtains the wtxid set when no proof-serving
gateway is available: download the raw block from any untrusted source and
build the witness tree locally. The input is untrusted: size and claimed
tx count are bounded up front, and each tx is parsed in place from a single
advancing offset (no per-tx tail copy), so parsing stays linear.
Parse a full serialized block (header + varint tx count + transactions). This is how an L3 verifier obtains the wtxid set when no proof-serving gateway is available: download the raw block from any untrusted source and build the witness tree locally. The input is untrusted: size and claimed tx count are bounded up front, and each tx is parsed in place from a single advancing offset (no per-tx tail copy), so parsing stays linear.