Streamline the MagicBlock Protocol v2 delegation account lifecycle

Goal

Streamline delegation and undelegation account lifecycle for MagicBlock Protocol v2 under #132.

Why

Reduce account churn and redundant delegation state, retain surplus lamports in a global vault while delegated, and charge the delegation fee at entry.

Scope

• On delegation, leave the rent-exempt minimum on the delegated account, move surplus lamports to a global vault, and account for their return.
• On undelegation, restore the vaulted lamports, zero account data, and assign the account back to the expected owner without closing and recreating it.
• Combine delegation record and metadata into one PDA; remove stored seeds only when the v2 flow no longer needs them.
• Charge the delegation fee exactly once during delegation, with no delegation-fee charge during undelegation.
• Cover lamport round trips, zero-and-assign behavior, the unified PDA layout, and fee timing in tests.
• Keep unrelated delegation behavior unchanged. Related: #59 tracks the existing zero-out-and-assign improvement.