feat: support other iso-8859 encodings in single-byte

[ChALkeR]

This brings in actual iso-8859-1 support, also iso-8859-9 and iso-8859-11
TextDecoder API is unchanged

[Copilot]

Pull request overview

This PR adds support for three ISO-8859 encodings (iso-8859-1, iso-8859-9, iso-8859-11) using their official Unicode.org definitions, which differ from WHATWG's label mappings to Windows encodings.

• Implements actual iso-8859-1 (Latin-1) as a direct Unicode subset rather than mapping to windows-1252
• Adds iso-8859-9 (Turkish) and iso-8859-11 (Thai) using official Unicode mappings
• Splits test suite to separately verify Unicode-based and WHATWG-based encoding behavior
• Optimizes iso-8859-1 encoding/decoding with fast paths using native Latin1 operations

Reviewed changes

Copilot reviewed 22 out of 22 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/single-byte.test.js	Adds new Unicode-based test suite alongside existing WHATWG tests; validates against official Unicode mapping tables
tests/encoding/fixtures/unicode/ISO8859/*.txt	Adds 16 official Unicode Consortium mapping files for ISO-8859 encodings (8859-1 through 8859-16)
tests/encoding/fixtures/README.md	Documents new Unicode.org fixture sources
fallback/single-byte.encodings.js	Refactors encoding maps to include all iso-8859-* variants (1-16); marks non-WHATWG variants
fallback/single-byte.js	Adds fast path for iso-8859-1 decoding using decodeLatin1()
single-byte.js	Adds non-WHATWG encodings to skipNative set; adds fast path for iso-8859-1 encoding
single-byte.node.js	Updates Node.js decoder to use latin1Slice() for iso-8859-1
README.md	Documents new encodings and clarifies WHATWG vs Unicode.org differences with examples

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[Copilot]

Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

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[Copilot]

Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated no new comments.

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