Professionalize API documentation, JSON contract, and tests

[domingogallardo]

Summary

• formalize the JSON export contract with ChatDumpPayload and getChatPayload(...)
• split WABackup into smaller extension-based units without breaking the public API
• rewrite consumer-facing docs and add a dedicated JSON contract document
• restructure the test suite into portable quick checks plus opt-in full regression coverage
• add CI workflows for quick verification and full regression runs

What changed

• Added ChatDumpPayload as the recommended Encodable full-chat export wrapper.
• Added getChatPayload(chatId:directoryToSaveMedia:) as a non-breaking convenience API.
• Expanded doc comments across the public API and normalized public-facing terminology to English.
• Reworked the README to cover installation, requirements, privacy, JSON export, media export, and error handling.
• Added Docs/JSONContract.md and JSON snapshot fixtures to make the serialized output explicit and testable.
• Split the test suite into smoke, error-handling, helper, JSON contract, and heavy regression suites.
• Added a generated sample backup fixture so swift test runs without the large private fixture.
• Preserved the large fixture regression tests behind SWIFT_WA_RUN_FULL_FIXTURE_TESTS=1.
• Added GitHub Actions for quick checks and scheduled/manual regression runs.

Verification

• swift test
• SWIFT_WA_RUN_FULL_FIXTURE_TESTS=1 swift test

Notes

• The full regression suite self-skips when the large local fixture is not present, which keeps CI portable while preserving the heavy regression path for environments that provide the fixture.
• Public compatibility is preserved; all API additions in this PR are additive.