fix(receipts): fix #58 by binding receipts to payload commitments

[Dopezapha]

Closes #58

PR Description

This PR fixes issue #58 by binding delivery receipts to encrypted payload commitments in the Soroban receipts contract.

Before this change, a receipt was keyed by message_id and stored sender, recipient, delivery time, and optional read time. That proved a message ID had a receipt, but it did not prove which encrypted payload the receipt referred to.

The delivery flow now records the message ID, payload hash, protocol version, sender, recipient, delivery timestamp, and optional read timestamp in the receipt state. The sender authorizes the delivery record with those commitment fields as part of the contract call. Once a receipt exists for a message ID, the contract rejects any attempt to reuse that ID with a different payload hash, protocol version, sender, or recipient. Reusing the same commitment also fails, so receipt state cannot be overwritten.

The read flow remains recipient-controlled. read(message_id) loads the existing receipt, requires authorization from the stored recipient, and only adds read_at; it does not change the payload commitment. A third party can call get(message_id) to verify receipt consistency against the known payload hash without seeing message content.

Changes Made

• Fixed contracts: bind delivery receipts to encrypted payload commitments #58 in contracts/soroban/receipts/src/lib.rs by adding message_id, payload_hash, and protocol_version to Receipt.
• Fixed contracts: bind delivery receipts to encrypted payload commitments #58 in contracts/soroban/receipts/src/lib.rs by changing delivered to accept and persist the payload commitment fields.
• Fixed contracts: bind delivery receipts to encrypted payload commitments #58 in contracts/soroban/receipts/src/lib.rs by rejecting duplicate IDs with changed commitments using CommitmentMismatch.
• Fixed contracts: bind delivery receipts to encrypted payload commitments #58 in contracts/soroban/receipts/src/lib.rs by preserving recipient-only control over read_at.
• Fixed contracts: bind delivery receipts to encrypted payload commitments #58 in contracts/soroban/receipts/src/lib.rs by adding typed receipt events for delivery and read transitions.
• Fixed contracts: bind delivery receipts to encrypted payload commitments #58 in contracts/soroban/receipts/src/lib.rs by adding tests for commitment persistence, duplicate handling, sender delivery auth, and recipient read auth.
• Updated contracts/soroban/receipts/README.md to document the new receipt interface and immutability behavior.