feat(sdk-coin-stx): add PoX-5 bond builders

[Doddanna17]

Summary

• Add PoX-5 contract constants and function validation.
• Add typed STX builders for bond registration, validator updates, early exit, STX-only staking, reward calculation, and claims.
• Add BTC/sBTC lockup Clarity value construction with SPV proof fields.
• Add factory parsing, sdk-core staking option support, and round-trip tests.

Verification

• Prettier: passed.
• ESLint: passed for changed files.
• Diff check: passed.
• Package tests/build are currently blocked before test collection by existing sdk-core io-ts and @bitgo/public-types/MPC dependency errors.

Ticket: SI-1486

[linear-code]

SI-1486

[github-actions]

⚠️ Unit tests are failing on Node 26.x (Current release line, non-blocking). This is not an LTS version yet, so it does not block merge, but it signals an incompatibility to fix before Node 26.x becomes LTS.

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