Feature/vault flex tier 35

[mijinummi]

PR Summary

feat(vault-flex): implement complete VaultFlex contract logic with unit tests

• Implements zero lock-up, 1.00x share allocation tier primitives
• Integrates authorization checks restricting access exclusively to the VaultRouter admin
• Enforces a minimum 1 USDC deposit threshold parameter
• Connects shared block-sequence checkpoints to prevent flash-loan yield manipulation
• Validates clean states with unit tests for deposit limits, yield payouts, and double-withdraw protection
• Closes [GF-02] [CONTRACT] VaultFlex: Full Implementation + Tests #35

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