feat: add Stellar deployment script and instructions

[thebabalola]

Added a versatile bash script for deploying all Stellar contracts to testnet, futurenet, or mainnet. Also added a README with build and deployment instructions. Closes #7.

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[thebabalola]

Hi @maintainer, I've added the Stellar deployment script and setup instructions. Ready for your review!

[truthixify]

The skeleton is right — accepting a network argument, deploying all four contracts in order, initializing stealth-sender with the announcer ID. Good baseline.

The blocker for merging is that the SDK and downstream consumers can't pick up the deployment automatically from echo output. Could you add:

1. JSON deployment manifest — write stellar/deployments/<network>.json with the schema the issue spec lays out:

   {
     "network": "futurenet",
     "deployer": "G...",
     "deployedAt": "2026-XX-XX...",
     "contracts": {
       "stealthAnnouncer": "C...",
       "stealthRegistry": "C...",
       "stealthSender": "C...",
       "wraithNames": "C..."
     }
   }

   This is the single most important deliverable — without it the SDK's getDeployment('stellar') can't read addresses.

2. Build + optimize step — currently the script assumes target/wasm32-unknown-unknown/release/*.wasm exists. Add cargo build --target wasm32-unknown-unknown --release and stellar contract optimize so it works from a clean checkout.

3. Idempotency guard — refuse to overwrite an existing deployments/<network>.json unless --force is passed. Mainnet sneakers will thank you.

4. --dry-run mode — print planned operations without spending fees. Useful for code review against deployed state.

Optional but nice: a verification step at the end that calls a read-only function on each fresh contract and confirms a sane response.

Once the JSON manifest piece lands the rest of the protocol (SDK, demo, Spectre) can start consuming the futurenet deployment automatically. Thanks @thebabalola — happy to re-review.