feat(shared): build fixed-point math, token client interfaces

[mijinummi]

PR Summary

feat(shared): build fixed-point math, token client interfaces, and checkpoint rules

• Embeds shared Rust crate within the root Cargo workspace tracking token metrics
• Eliminates fee calculation truncation tracking under 5 USDC boundaries (Resolves M-02)
• Implements delayed ledger checkpoint lookups to prevent same-ledger yield exploits (Resolves M-01)
• Standardizes cross-contract Client traits across VaultRouter and Strategy targets
• Configures storage-backed allowance controls for XLM, EURC, and AQUA entries
• Resolves issue
  closes [GF-01] [CONTRACT] Shared Contract Infrastructure (fixed-point math, USDC interface, share-checkpoint) #34

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