Add Soroban footprint comparison example

[octaviapeter35-beep]

closes #241
closes #242
closes #243
closes #244

Summary

• add a new Soroban footprint comparison example that compares multiple invocation results
• separate read-only and read-write entries
• detect shared, added, removed, and access-mode changes
• report the smaller footprint and JSON output support
• register the example in the runner and document it in the README

Verification

stellar-sdk-example-hub@1.0.0 test
jest --coverage --runTestsByPath tests/examples.test.ts --runInBand --coverage=false

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