feat: API stabilization and breaking change review for v1.0

[satoshai-dev]

Summary

Review the full public API surface before v1.0 and make any necessary breaking changes while we still can.

Scope

• Review testWithStacks() config shape — is it ergonomic? Any footguns?
• Review StacksFixture interface — naming, method signatures, return types
• Review re-exports from @stacks/transactions — are we exposing too much/too little?
• Ensure all types are properly exported and documented
• Consider if PlaystacksConfig union (PrivateKeyConfig | MnemonicConfig) is clear enough
• Review error types and error messages
• Write migration guide if any breaking changes are made

Milestone

v1.0