Use SMALL_ATOM_UTF8_EXT and ATOM_UTF8_EXT for safer atom encoding

[sile]

Fix #13

[Copilot]

Pull Request Overview

Updates atom encoding to use the UTF-8 atom encoding tags (SMALL_ATOM_UTF8_EXT and ATOM_UTF8_EXT) instead of the deprecated ASCII-only atom encoding tag (ATOM_EXT). This ensures proper handling of non-ASCII characters in atom names and follows current Erlang Term Format conventions.

• Changes atom encoding logic to use length-based encoding selection instead of ASCII detection
• Updates all test assertions to reflect the new encoding format
• Updates documentation examples to use the new encoding

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/codec.rs	Core encoding logic updated to use UTF-8 atom encoding based on length
tests/lib.rs	Test assertions updated to match new encoding format
src/lib.rs	Documentation examples updated to reflect new encoding

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