Implement SDK transaction readiness validator

[El-swaggerito]

Description

Difficulty: Expert

Type: Feature

Summary

Implement a transaction readiness validator for PocketPay SDK payment flows.

Current Behaviour

Payment transactions may be built without a consistent readiness check for destination, amount, asset, memo, network, account balance, and fee assumptions.

Expected Behaviour

The SDK should expose a typed readiness result that blocks invalid transactions before signing or submission.

Suggested Implementation

Create readiness types, validator utilities, error mapping, fixtures, tests, and documentation.

Files or Areas Likely Affected

• src/transactions/
• src/payments/
• src/errors/
• src/types/
• tests/
• docs/

Acceptance Criteria

• Transaction readiness validator is implemented.
• Destination, amount, asset, memo, and network are validated.
• Invalid readiness states return typed errors.
• Payment helpers consume the validator where appropriate.
• Tests cover valid, invalid, and edge-case states.
• Documentation explains readiness limitations.

Additional Notes

This should be a meaningful SDK implementation, not a docs-only change.