BIP352: Add intermediate vector material for silent payments

This PR adds intermediate computation vectors to the BIP-352 reference implementation to improve development experience and code clarity.

Changes

• Updated reference.py to validate intermediate values during silent payment computations
• Added comprehensive intermediate vector material to send_and_receive_test_vectors.json
• Applied labels suggestion by @fifalodm to scanning

Benefits

• secp256k1 library no longer needs to handle silent payments specific code, intermediate values can be used instead
• Other projects can use intermediate values to clarify computations during development and debugging

This change makes it easier for implementers to understand and debug silent payment computations by providing visibility into intermediate calculation steps.