Refactor secret conditions

[callebtc]

More rigid structure in validation and full documentation of all steps for P2PK input and output checking, and for HTLC input checking.

[codecov]

Codecov Report

Attention: 78 lines in your changes are missing coverage. Please review.

Comparison is base (0490f20) 68.17% compared to head (305598b) 68.16%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #350      +/-   ##
==========================================
- Coverage   68.17%   68.16%   -0.02%     
==========================================
  Files          65       65              
  Lines        4817     4821       +4     
==========================================
+ Hits         3284     3286       +2     
- Misses       1533     1535       +2     

Files	Coverage Δ
cashu/core/secret.py	100.00% <100.00%> (ø)
cashu/wallet/htlc.py	88.46% <ø> (ø)
cashu/wallet/p2pk.py	100.00% <100.00%> (ø)
cashu/core/htlc.py	63.63% <0.00%> (ø)
cashu/core/p2pk.py	59.61% <87.50%> (+1.61%)	⬆️
cashu/core/base.py	93.91% <0.00%> (+0.65%)	⬆️
cashu/mint/conditions.py	21.55% <15.11%> (-0.07%)	⬇️

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