Identify all instances of Wit/2.0 "single-withdrawer" assumptions in the codebase

[aesedepece]

Wit/2.0 enforced a 1:1 relationship between stake entries and withdrawer addresses (i.e. each validator has a single associated withdrawer). In preparation for delegation support in Wit/2.1, we need to identify all parts of the codebase where this "single-withdrawer" assumption is present.

Before lifting this restriction, it's critical to audit the business logic across the staking, rewards, and withdrawal flows to ensure they behave correctly under a one-to-many relationship (i.e. multiple withdrawers per validator).

This task includes:

• Systematically identifying all places in the code that rely on or enforce the 1:1 withdrawer assumption
• Documenting each instance with context and potential implications
• Proposing how to approach each case:
  • Remove condition or assumption
  • Replace with an iteration over multiple entries
  • Fully refactor data structures or logic

This issue will lay the groundwork for safely extending the protocol to support delegated staking while preserving correctness and backward compatibility where necessary.