feat(wallet): add Wallet::sign_psbt<K> wrapping bitcoin::Psbt::sign

[EliteCoder18]

Description

Related to #331

Adds Wallet::sign_psbt<K>, a thin wrapper around bitcoin::Psbt::sign that passes the wallet's internal secp256k1 context to the caller's GetKey implementor (e.g. Xpriv, a hardware wallet adapter).

Without this, callers driving signing themselves via the bitcoin crate must construct their own Secp256k1<All> — setup the wallet already owns.

Notes to the reviewers

• BIP32 derivation prerequisite: bitcoin::Psbt::sign reads bip32_derivation fields to locate child keys. A freshly built PSBT lacks these. Callers must first run wallet.sign(&mut psbt, SignOptions { try_finalize: false, ..Default::default() }) to populate them. Documented in rustdoc.
• sign vs sign_psbt: Wallet::sign uses BDK's internal signer infrastructure (InputSigner/TransactionSigner). sign_psbt delegates entirely to bitcoin::Psbt::sign; callers supply their own GetKey. Both methods coexist without conflict.
• Descriptor-aware variant deferred: The KeyMapWrapper variant (rust-miniscript#867) is postponed to 4.0 per maintainer direction.

Changelog notice

• Added Wallet::sign_psbt<K> wrapper around bitcoin::Psbt::sign using the wallet's internal secp256k1 context.

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran just p before pushing

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.07%. Comparing base (fb7681a) to head (ce3d189).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #438      +/-   ##
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+ Coverage   80.04%   80.07%   +0.03%     
==========================================
  Files          24       24              
  Lines        5336     5345       +9     
  Branches      242      242              
==========================================
+ Hits         4271     4280       +9     
  Misses        987      987              
  Partials       78       78              

Flag	Coverage Δ
rust	80.07% <100.00%> (+0.03%)	⬆️

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[ValuedMammal]

Concept ACK. I've not had an extensive look at the implementation.

Callers must first run wallet.sign()

sign_psbt should be written in a way that assumes the PSBT has been updated and totally independent of wallet.sign(). For the tests you can create the PSBT to sign by parsing it from a string in base64.

[EliteCoder18]

@ValuedMammal Thanks for the feedback!

I've updated the tests to use a base64 PSBT with pre-populated bip32_derivation, so sign_psbt is now fully independent of wallet.sign.

Would appreciate a review when you get a chance.