Add streaming encoder and decoder API

[tortxof]

Summary

• Add streaming io.WriteCloser implementations for encoding/decoding (NewEncoder, NewDecoder)
• Refactor CLI tools (z85encode, z85decode) to use the new streaming API
• Add comprehensive tests for CLI tools and streaming round-trip functionality
• Add GitHub Actions workflow for automated testing
• Update README with streaming API documentation and usage examples

Test plan

• All existing unit tests pass
• New streaming round-trip tests added
• Fuzz test FuzzStreamRoundTrip validates chunked encode/decode
• CLI tool tests verify end-to-end behavior
• Verify GitHub Actions workflow runs on PR

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