Align subtrees with updated specs

[phbnf]

This commit updates the Merkle tree proof generation, verification, and reference implementations to align with recent specification changes in ietf-plants-wg/merkle-tree-certs:

* **PR #274 (`find_subtrees` for size <= 1 intervals):**
  * Updated `FindSubtrees` in proof.go to return two subtrees (`[start, end)` and `[end, end)`) when `end - start <= 1`, removing the previous special case that returned only one subtree when interval size

was <= 1.

* **PR #275 (0-length / empty subtrees):**
  * **Proof Generation & Validity:** Updated `isSubtreeValid` in proof.go and tree.go to treat `start == end` as a valid 0-length subtree. `SubtreeConsistencyProof` now returns an empty proof slice

([][]byte{}) when start == end.
* Proof Verification: Updated VerifySubtreeConsistency in verify.go to check if start == end upfront, verifying end <= size, len(proof) == 0, and subRoot == hasher.EmptyRoot() without
duplicate subtree validity calls or comparing against parentRoot. Updated RootFromSubtreeConsistencyProof to return hasher.EmptyRoot() when start == end && size == 0.
* Reference Implementation: Clarified preconditions in refSubtreeConsistencyProof in reference_test.go to explicitly handle start == end returning nil (empty proof).
* Tests & Vectors: Updated test loop bounds across proof_test.go, tree_test.go, and vectors_test.go to use inclusive upper limits (range ... + 1) to cover right edges and empty subtrees. Updated
rolling SHA-256 test digests to match PR #275.
* Probe Generator: Added static test vectors for empty subtrees in main.go and updated generated JSON testdata in testdata/subtreeconsistency/.

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