feat(sdk,shared): add SDK README usage example and network/address helpers

[starfishkrq]

Summary

closes #10
closes #11

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Issue #10 — docs(sdk): add packages/sdk/README.md usage example

• Full LumioClient construction example using NETWORKS.testnet and a ContractIds map (mirrors the real shape from sdk.test.ts)
• Read methods section showing mock return values for all four clients
• Write methods section showing NotImplementedError handling
• Individual contract client direct-use example
• Types section documenting @lumio/shared re-exports
• Scaffold/stub caveat stated prominently at the top
• Root README.md package table now links to packages/sdk/README.md, packages/shared/README.md, and packages/ui/README.md

Issue #11 — feat(shared): add getNetwork() and isValidAddress()

packages/shared/src/utils.ts

• getNetwork(name: NetworkName): NetworkConfig — resolves a well-known network config by name
• isValidAddress(value: string): boolean — lightweight format guard for Stellar strkeys (56 chars, G/C prefix, base32 A–Z/2–7 charset); documents that it is a shape check only, not a checksum or on-chain verification

Both functions are automatically exported via the existing export * from "./utils" in index.ts.

packages/shared/src/utils.test.ts

• 4 tests for getNetwork (testnet/futurenet/mainnet configs, same reference as NETWORKS)
• 7 tests for isValidAddress (valid G and C keys, too short, too long, wrong prefix, non-base32 chars, empty string)

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Validation

• pnpm lint ✅
• pnpm typecheck ✅
• pnpm build ✅
• pnpm test ✅ — 36 tests in @lumio/shared (up from 25), all passing

[coderabbitai]

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