[FIP-442] Optional Early Exit for the Legacy crvUSD/L-FRAX Convex FRAX Vault (4% Fee)

Authors: Mockingbird

Summary: This proposal introduces a one-time optional early exit program for the legacy crvUSD/L-FRAX Convex FRAX vault.

Eligible lockers may exit before lock expiry by paying a fixed 4% fee on the amount exited. The 4% fee is the sole economic condition of this optional early exit path.

This proposal applies only to this specific legacy vault.

Background & Motivation: The crvUSD/L-FRAX Convex FRAX vault is a legacy route after the FRAX to frxUSD stablecoin transition.

This case is specific because:

1. Long-duration locks were entered more than 2 years ago, before frxUSD existed as the supported successor route.

2. Returns were only reasonable during the first few months, then remained very low for a long period, and have been 0% since around August 2025.

3. Th