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

[robwoodgate]

Description

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-20/29 mint quote signatures, and reuses their helpers.

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Notes to the reviewers

• The mint accepts the v1 and current message formats (sig_all_msgs_to_verify), and wallet signing produces both, so requests verify on mints on either side of the upgrade. The pre-0.14 format is neither signed nor accepted: it does not commit to C values or output amounts, and test_sig_all_should_reject_if_the_output_amounts_are_swapped covers this.
• valid_signatures_any_msg counts each pubkey at most once across formats, so a multi-format witness cannot trip DuplicateSignature.
• The witness length cap is split from the secret cap (MAX_PROOF_WITNESS_LEN, 8192 bytes) so multisig witnesses carrying one signature per format per signer still fit.
• Canonical vectors from the spec's tests/11-test.md are pinned in the nut11 test module.
• Ran on stable 1.96 in a container: cargo test -p cashu --features mint,wallet,nostr, cargo test -p cdk --lib, cargo clippy -p cashu --all-targets, cargo fmt --check.

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Suggested CHANGELOG Updates

CHANGED

• SIG_ALL verification accepts the length-framed v1 message alongside the current format, counting unique pubkeys with valid signatures and ignoring signatures that do not verify
• Proof witness length is capped separately from secret length (8192 bytes)

ADDED

• NUT-11 v1 length-framed SIG_ALL message; sign_sig_all signs all accepted message formats

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Checklist

• I followed the code style guidelines
• I ran just quick-check before committing (ran the equivalent commands listed above on stable in a container)
• If the Wallet API was modified (added/removed/changed), I have reflected those changes in the FFI bindings (crates/cdk-ffi) — not modified

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.01%. Comparing base (8716e53) to head (c64c544).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2232      +/-   ##
==========================================
+ Coverage   76.99%   77.01%   +0.01%     
==========================================
  Files         379      379              
  Lines       96708    96823     +115     
==========================================
+ Hits        74460    74566     +106     
- Misses      22248    22257       +9     

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[asmogo]

@cdk-bot review

[thesimplekid]

@cdk-bot review

Seem like the bot took a nap last night. Should be back up and reviewing now

[robwoodgate]

Removed the legacy signing format this PR had re-added.