tlog: Add MTC subtree primitives

[beautifulentropy]

Implement the generation and verification of MTC subtree consistency proofs, which prove that an aligned interval of the log (a subtree) belongs to the tree under a given root hash. The MTC draft defines these in section 4.

x/mod's sumdb/tlog provides the hash primitives and the stored-hash reader these functions use. Its proofs cover only two cases: an inclusion proof for a single entry and a consistency proof for a prefix of the log. MTC requires a proof for any subtree; x/mod has no such proof.

Export four functions:

• SubtreeHash computes an RFC 6962 Merkle Tree Hash over a list of leaf hashes.
• ValidSubtree reports whether a given interval is a valid MTC Subtree.
• SubtreeConsistencyProof generates an MTC Subtree Consistency Proof.
• VerifySubtreeConsistency verifies an MTC Subtree Consistency Proof against a given root hash.

A note to reviewers: this package and its tests were written with assistance from Claude Opus 4.8 and Fable 5. That being said, it has been reviewed and extensively revised by myself before submission.