URI-based proof payload locations with blob publication verification

[mfw78]

Proof { uint256 location; bytes data } is a convention-only registry with an unspecified payload — an indexer cannot go root → leaf set.

Implement docs/discovery.md §3:

• Proof { string[] uris; bytes32[] blobVersionedHashes } — URI mirrors (opaque on-chain) + EIP-4844 publication verification (blobhash() loop, BlobNotAttached)
• setRoot(bytes32(0)) = explicit clear; proof must be empty (ProofDataMalformed)
• normative tree-construction test vectors (leafEncoding: "v1")