feat(compile): compile taproot descriptor with randomized unspendable internal key

[va-an]

Description

Addresses #218 (part 1/2).

Implements randomization of the unspendable internal key for taproot descriptors. This is the first part of #218, which consists of two parts:

1. This PR: Randomize unspendable internal key for taproot descriptor
2. Follow-up: Add verification command to ensure internal key is derived from NUMS key

Split into separate PRs for easier review and iteration, and to allow independent discussion of the verification command implementation, as one of the possible approaches could introduce breaking changes.

Notes to the reviewers

The compile command now returns an additional r field for taproot descriptors (-t tr), containing the randomly generated internal key. Each compilation will produce a different internal key instead of using a fixed NUMS key.

Example output for taproot (first execution):

-> % bdk-cli compile "pk(A)" -t tr
{
  "descriptor": "tr(2dd09dd0355f4b2d5a4886de599786f3c0211652373221c87aba1cd1f7f1e593,pk(A))#anvu48aj",
  "r": "275a58827bd0ad459d6f92e083ddc3d99a03076155691680eb8f3b06380cdcfd"
}

Same descriptor compiled again produces different r and internal key:

-> % bdk-cli compile "pk(A)" -t tr
{
  "descriptor": "tr(801078f69dae7d95631723d4d13e6c32911633d227dcfc24c6b7e32e1e533e6c,pk(A))#f79rr82j",
  "r": "5e3ac63bb20d6a4bfff645279cc63a7472e18066da8826b13cbcb23aecb5c401"
}

Other descriptor types remain unchanged:

-> % bdk-cli compile "pk(A)" -t sh
{
  "descriptor": "sh(pk(A))#k80zhe7s"
}

Tests for compile command have been moved from handlers.rs to the tests directory. Since taproot descriptors now generate a random internal key on each invocation, the test for the compile command has been simplified. I plan to enhance this test in a follow-up PR once the verification command is implemented.

Changelog notice

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

New Features:

• I've added tests for the new feature
• I've added docs for the new feature
• I've updated CHANGELOG.md

[coveralls]

Pull Request Test Coverage Report for Build 19500901936

Details

• 18 of 27 (66.67%) changed or added relevant lines in 1 file are covered.
• 4 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-2.7%) to 9.402%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/handlers.rs	18	27	66.67%

Files with Coverage Reduction	New Missed Lines	%
src/handlers.rs	4	11.69%

Totals
Change from base Build 19253249526:	-2.7%
Covered Lines:	129
Relevant Lines:	1372

---

💛 - Coveralls