Implement typed storage helpers

[TUPM96]

Closes #11

Summary

• Move storage keys into a typed storage module with helper functions for escrows, vendor/buyer indexes, escrow counter, admin address, and fee config.
• Refactor escrow flows to use the storage helpers instead of direct business-logic storage access.
• Add vendor/buyer escrow index view methods and tests, including updated Soroban snapshots.

Validation

• cargo fmt --check
• cargo test (14 passed; existing deprecation warning remains for register_stellar_asset_contract in tests)
• cargo build --target wasm32v1-none --release
• git diff --check

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