Align subtrees with updated specs

[phbnf]

Updates subtree consistency proof generation, verification, and test vectors to align with recent changes in ietf-plants-wg/merkle-tree-certs:

• ietf-plants-wg/merkle-tree-certs - PR#275
• ietf-plants-wg/merkle-tree-certs - PR#274

Specifically:

• Empty subtrees: start == end is now treated as a valid 0-length subtree. Subtree consistency proofs for empty subtrees return an empty array and verify that the subtree root equals the empty tree hash (HASH()), without depending on the parent tree's root.
• FindSubtrees edge case: Intervals of size <= 1 now return two subtrees ([start, end) and [end, end)) instead of one.
• Tests & Probes: Updated test loops to include empty subtrees (start == end), updated reference vectors to match the new rolling SHA-256 digests, and added empty subtree test cases to proofgen.

[codecov]

Codecov Report

❌ Patch coverage is 31.57895% with 39 lines in your changes missing coverage. Please review.
✅ Project coverage is 47.16%. Comparing base (98f61d9) to head (accbc13).

Files with missing lines	Patch %	Lines
cmd/proofgen/main.go	0.00%	29 Missing ⚠️
proof/verify.go	42.85%	7 Missing and 1 partial ⚠️
testonly/tree.go	50.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #252      +/-   ##
==========================================
- Coverage   48.01%   47.16%   -0.85%     
==========================================
  Files           8        8              
  Lines         933      971      +38     
==========================================
+ Hits          448      458      +10     
- Misses        476      502      +26     
- Partials        9       11       +2     

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