Add request identity to v2 standalone actions

@snawaz is already working on this.

Since Sep 17, 2026.

### Goal

Give v2 standalone actions protocol-level request identity through an opaque nonce or request ID in the wrapper and its APIs. Distinct requests use distinct identities; retries and recovery retain the original identity.

### Why

Two standalone-action requests can produce identical base-chain transactions when the validator authority, forwarded action data, and recent blockhash match. They then share a signature, so confirming the second request can merely confirm the first execution.

MBV currently distinguishes transaction messages with a noop containing the scheduled intent ID. This leaves user data unchanged but does not give standalone actions protocol-level request identity.

### Scope

- Include the identity in the transaction message without forwarding it to the destination program or changing the destination's action data.
- Define nonce ownership, validation, replay semantics, and compatibility for callers and instruction encoding. Nonce bytes alone do not guarantee execution once, particularly when a retry uses a new blockhash. Specify how repeated requests are recognized or constrained and what clients can rely on when reconciling an uncertain submission.
- Keep commit/action and undelegate/action identity tied to their existing commit scope. This is v2 work under #132, not a v1 backport.

Keeping MBV's intent-ID noop or varying a compute-budget instruction are transaction-level alternatives, not substitutes for defined replay semantics or additional implementation requirements.

[magicblock-validator#1280](https://github.com/magicblock-labs/magicblock-validator/issues/1280) owns MBV adoption and removal of noop padding once the replacement provides the required guarantees. [magicblock-validator#1702](https://github.com/magicblock-labs/magicblock-validator/issues/1702) owns durable progress and reconciliation of submitted transactions after interruption.