Feature/731 require sufficient shares

[rabsqueen]

Summary

Implements the core compound_fees functionality for the Yield-Bearing Fee Vaults epic.

The new functionality moves pending protocol fees from the fee contract into the vault pool and adds the compounded tokens to the vault's underlying balance.

This establishes the core mechanism required for protocol fees to be reinvested into the vault rather than remaining idle in the fee contract.

What Changed
Fee Compounding

Implemented the compound_fees function to:

Identify pending fees held by the fee contract.
Pull the available tokens from the fee contract.
Transfer the tokens into the vault pool.
Increase the vault's underlying balance by the compounded amount.

closes #731
closes #722
closes #728
closes #729

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