feat(p25-crypto): add get_invoice_fingerprint using SHA-256 host function

Protocol 25 (X-Ray) and Protocol 26 expose crypto host functions including
SHA-256 for deterministic content hashing.

- get_invoice_fingerprint(invoice_id) returns BytesN<32>
- Commits to: invoice_id, deadline, recipient_count, total_amount
- Deterministic — same invoice always produces same hash
- Tamper-evident — any change to invoice terms changes the hash
- Useful for off-chain verification, receipts, and audit trails
- Test: test_invoice_fingerprint_is_deterministic

Author: MarcusDavidG