feat: Stellar asset pair liquidity, DEX VWAP, Soroban TTL renewal, contract address derivation

[abore9769]

Closes #790, closes #791, closes #792, closes #793

Summary

• Build Soroban Contract Address Derivation Utility with WASM Hash Determinism #790 — deriveContractAddress / verifyContractAddress added to packages/stellar/src/soroban.ts supporting all three Soroban preimage types (from-address, from-wasm-hash, from-asset)
• Implement Stellar DEX Price Feed Aggregation with Outlier Detection and VWAP Calculation #791 — VWAP calculation, 3σ outlier detection, and ledger-close triggered price feed in packages/stellar/src/dex-price-feed.ts
• Design Soroban TTL Management Automation with Ledger Sequence-Aware Renewal #792 — AutomaticTTLRenewer class in packages/stellar/src/soroban-ttl-manager.ts: monitors entries < 1000 ledgers, renews at 500 ledgers, batches into one tx, emits alert on failure
• Build Stellar Asset Pair Validation with Cross-Asset Bridge Liquidity Verification #793 — checkBridgeLiquidity() in apps/backend/src/lib/stellar/validate-asset-pairs.ts: checks Horizon order book depth (min $1,000 each side), 5-minute cache

Tests

• 9 tests for Build Soroban Contract Address Derivation Utility with WASM Hash Determinism #790 (soroban-address-derivation.test.ts)
• 14 tests for Implement Stellar DEX Price Feed Aggregation with Outlier Detection and VWAP Calculation #791 (dex-price-feed.test.ts)
• 12 tests for Design Soroban TTL Management Automation with Ledger Sequence-Aware Renewal #792 (soroban-ttl-renewal.test.ts)
• 5 tests for Build Stellar Asset Pair Validation with Cross-Asset Bridge Liquidity Verification #793 (appended to validate-asset-pairs.test.ts)

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