Update SignSubtree to match updated specs

[AlCutter]

This PR:

• Updates the MLDSA SignSubtree implementation to match the updated tlog-cosignature and MTC specs, returning raw MLDSA cosignatures rather than note-style signatures.
• Expands the test coverage to include those checks listed in the PLANTS RFC, and adds an interop test for MLDSA note cosigs with Torchwood.
• Introduces a few small tidy-ups.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 90.62500% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.53%. Comparing base (fa00c16) to head (9bd54a2).
⚠️ Report is 197 commits behind head on main.

Files with missing lines	Patch %	Lines
note/note_cosigv1.go	90.62%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #287      +/-   ##
==========================================
+ Coverage   82.57%   88.53%   +5.96%     
==========================================
  Files           5       10       +5     
  Lines         241     1012     +771     
==========================================
+ Hits          199      896     +697     
- Misses         30      116      +86     
+ Partials       12        0      -12     

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[roger2hk]

Do you want to rename hash to root like what you did in SubtreeVerifier interface?