feat(aztec-nr): let contracts choose the message tag derivation

[nchamo]

Motivation

The handshake registry delivers its own notes as a self-send, tagged through the generic on-chain tagging path. That path references STANDARD_HANDSHAKE_REGISTRY_ADDRESS, so the registry embeds its own address in its own bytecode. A contract's deploy address is a hash of its bytecode, so the pinned address never converges: re-pinning moves the bytecode, which moves the address.

Building on #24387, a contract can now fix its tag derivation at compile time. When the registry fixes the address-derived derivation, the handshake-registry references fold out of its bytecode via dead-code elimination, the self-reference disappears, and the address converges, all without changing the emitted tag.

The change

• A contract selects its tag derivation via the on-chain delivery builder's via_* methods, honored over the wallet's default resolution:
  • via_non_interactive_handshake()
  • via_address_derived_secret()
• The on-chain builder is split into OnchainUnconstrainedDelivery and OnchainConstrainedDelivery; the address-derived derivation is exposed only on the unconstrained builder, since it cannot back constrained delivery.
• The default (no via_* call) is unchanged: reuse a registered handshake, otherwise the wallet-resolved strategy.
• Re-pins the standard contracts; the registry address now converges (idempotent re-pin).

[vezenovm]

looks good, mostly nits, approving as I leave to you to decide which you want to adopt