Multipath descriptor support (BIP 389)

[schjonhaug]

Description

Key Features:

• New API: Wallet::create_multipath(descriptor) following the same pattern as create() and create_single()
• BIP 389 compliance with exactly 2-path validation (receive and change)
• Robust validation with clear error messages
• Pattern consistency with existing wallet creation methods

Usage Example:

let multipath_desc = "wpkh([9a6a2580/84'/1'/0']tpub.../‹0;1›/*)";
let wallet = Wallet::create_multipath(multipath_desc)
      .network(Network::Testnet)
      .create_wallet_no_persist()?;

// Automatically creates separate receive and change descriptors
let receive_addr = wallet.peek_address(KeychainKind::External, 0);  // Uses path /0/*
let change_addr = wallet.peek_address(KeychainKind::Internal, 0);   // Uses path /1/*

Notes to the reviewers

Design Decisions:

1. Pattern Consistency: Uses make_multipath_descriptor_to_extract() helper following the same pattern as existing make_descriptor_to_extract() function
2. Lazy Evaluation: Descriptor parsing only happens when needed during wallet creation, not during parameter setup - this maintains performance and follows Rust's lazy evaluation patterns
3. Strict Validation: Only allows exactly 2-path multipath descriptors to ensure proper receive/change separation
4. API Consistency: The create_multipath() method returns CreateParams just like create() and create_single(), maintaining the fluent builder pattern

Implementation Notes:

• The function calls make_multipath_descriptor_to_extract() twice (once for receive, once for change) which is intentional and follows the established pattern of lazy evaluation
• Validation occurs in the descriptor extraction closures, providing clear error messages when descriptors are actually processed
• All existing functionality remains unchanged - this is a purely additive feature

Changelog notice

Added:

• Wallet::create_multipath() method for creating wallets from BIP 389 multipath descriptors
• CreateParams::new_multipath() for multipath descriptor parameter creation
• Support for 2-path multipath descriptors with automatic receive/change separation
• Enhanced validation for multipath descriptors with descriptive error messages

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

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

Closes #11

[coveralls]

Pull Request Test Coverage Report for Build 16072648456

Details

• 111 of 112 (99.11%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 84.947%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
wallet/src/descriptor/error.rs	0	1	0.0%

Totals
Change from base Build 16052072493:	0.2%
Covered Lines:	6687
Relevant Lines:	7872

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💛 - Coveralls

[ValuedMammal]

Thanks for working on this @schjonhaug.

[schjonhaug]

Glad to contribute, @ValuedMammal. Let me know if there’s anything I can improve!

[notmandatory]

Thanks for taking this one on, overall your PR looks great. I especially like the detailed docs and new validation to ensure only two indexes are used.

I have two small suggestions and you should do a rebase and then I think it's ready to merge.