Add support for configurable BIP39 mnemonic word counts

[coreyphillips]

Summary

This PR introduces support for generating BIP39 mnemonics with configurable word counts - 12, 15, 18, 21, or 24 words.
The word_count parameter is optional and defaults to 24 words (256-bit entropy).

Motivation

Different use cases benefit from different mnemonic lengths:

• 12 words: Simplified UX and easier memorization for users (default in Bitkit)
• Supporting configurable lengths increases flexibility for downstream applications integrating with this library.

Changes

• Added WordCount enum (Words12, Words15, Words18, Words21, Words24)
• Updated generate_entropy_mnemonic() to accept an optional word_count parameter
• Default: 24 words when no parameter is provided
• Updated UDL bindings to expose the new enum and function signature
• Updated tests to cover all variants and validate entropy mapping

Breaking Changes

⚠️ This is a breaking change - generate_entropy_mnemonic() now requires a word_count parameter.

Users will need to update their code from:

let mnemonic = generate_entropy_mnemonic();

To:

let mnemonic = generate_entropy_mnemonic(None);  // Uses default 24 words
// Or specify a word count:
let mnemonic = generate_entropy_mnemonic(Some(WordCount::Words12));

Testing

• Verified default behavior (24 words)
• Added tests for all word count variants
• All tests pass (cargo test)

Summary was co-created with AI

[ldk-reviews-bot]

I've assigned @tnull as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

[tnull]

Thanks, I think it makes sense to add this option.

Note that we appreciate human-written descriptions and when AI agent co-authorship is disclosed in the commit message.

[tnull]

Hmm, rather than dealing with different entropy byte lengths here, can we just use Mnemonic::generate which allows us to specify the word count? Having an enum for it still makes sense (actually we might want to switch to that in the bip39 API rust-bitcoin/rust-bip39#97).

[coreyphillips]

The latest commit here passes the WordCount directly to Mnemonic::generate() as suggested. However, I'm not sure how you may feel about the function name generate_entropy_mnemonic after this change. I've left it as is for now.

I've also created a PR adding the enum WordCount to rust-bip39 here.

[tnull]

LGTM, please squash the second commit so we can land this.

[coreyphillips]

Squashed as requested.