feat: verify and sign the length-framed NUT-11 SIG_ALL message

[robwoodgate]

Implements the length-framed NUT-11 SIG_ALL message aggregation (Cashu_SigAllSig_v1) proposed in cashubtc/nuts#404. The new format matches the domain separation and length framing already used for NUT-20 mint quote signatures.

• The mint verifies the v1 and current message formats, counts unique public keys with valid signatures, and ignores signatures that do not verify (including malformed ones, which previously aborted the verification loop)
• The wallet signs v1, current and legacy, so its transactions verify on mints at any upgrade stage
• Melt verification passes the quote id instead of a prebuilt message string; each format binds the quote per its own rules
• Canonical vectors from the spec's tests/11-test.md are pinned in tests/nuts/test_nut11_test_vectors.py

Draft until cashubtc/nuts#404 settles.

[codecov]

Codecov Report

❌ Patch coverage is 95.55556% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.60%. Comparing base (47726cc) to head (0111594).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
cashu/mint/conditions.py	85.71%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1088      +/-   ##
==========================================
+ Coverage   74.56%   74.60%   +0.04%     
==========================================
  Files         115      115              
  Lines       13167    13189      +22     
==========================================
+ Hits         9818     9840      +22     
  Misses       3349     3349              

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