Guard against over-length origins

[AlCutter]

This PR enforces the requirement that log (and mirror) origins are no longer than 255 bytes.

This is implicitly required by tlog-cosignature, which uses uint8 when encoding origin lengths in the MLDSA cosigned message.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.79%. Comparing base (fa00c16) to head (7ff1861).
⚠️ Report is 172 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #260      +/-   ##
==========================================
- Coverage   82.57%   76.79%   -5.79%     
==========================================
  Files           5        8       +3     
  Lines         241      698     +457     
==========================================
+ Hits          199      536     +337     
- Misses         30       89      +59     
- Partials       12       73      +61     

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