feat(soroban): add buildContractUpgrade utility with WASM validation …

[funds0033-cmyk]

Closes #120
What was done
Implemented buildContractUpgrade(contractId, newWasmCode) to provide a safe, validated helper for preparing Soroban contract upgrade transactions.
Changes

buildContractUpgrade — added to src/soroban/index.ts, validates contract ID, WASM size, magic bytes, and WASM version before building the transaction
XDR output — builds a Soroban upgrade invoke operation using the SHA-256 hash of the new WASM, returns SorokitResult (XDR)
Public API export — exported from src/index.ts
Tests — added to src/tests/soroban.test.ts covering valid upgrade XDR output and invalid WASM rejection

Verification

npx tsx runtime check passed — generated XDR parses back to upgrade, includes the expected WASM hash, and correctly rejects invalid code with TX_BUILD_FAILED ✅

Notes

npm run typecheck failures are pre-existing in unrelated files — no errors in src/soroban/index.ts
npx vitest collection fails due to a pre-existing require("../soroban/storageSlot") module-resolution issue unrelated to this implementation

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