feat(escrow): add Soroban escrow deploy script and docs\n\nCloses #126

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Adds a helper script to build the escrow Soroban contract WASM and a deployment guide for deploying to the Stellar Testnet. Enables end-to-end wiring for the frontend/backend to call the on-chain escrow contract.

Changes:

Added: contracts/deploy_escrow_testnet.sh — build helper and optional auto-deploy.
Added: docs/soroban-escrow-deployment.md — prerequisites, CLI examples, and integration notes.
Updated: README.md — link to deployment doc.
Context: Contract implementation exists at lib.rs. JS stub to replace: deploy.ts.
How to test:
Run the build helper locally (requires rust, wasm32 target, and soroban CLI):

Notes / Next steps:

The current JS stub at deploy.ts should be replaced with a TypeScript deployer that uploads WASM and invokes initialize via @stellar/stellar-sdk or Soroban RPC.
The branch is feature/escrow-deploy-126; this PR moves the deliverables off main so they can be reviewed.
Recommend reviewers: contracts/Soroban maintainers and a backend owner familiar with Stellar integration.
Closes #126

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