Add comprehensive tests for char module

[Copilot]

The char module lacked sufficient test coverage for character conversion methods, Unicode handling, and UTF-16 encoding behaviors.

Tests Added

• Character conversions: Int::to_char(), Int::unsafe_to_char(), Char::to_int(), Char::to_uint()
• UTF-16 encoding: Char::utf16_len() for BMP and supplementary plane characters, Char::is_bmp()
• Surrogate detection: Int::is_surrogate() for UTF-16 surrogate pairs (0xD800-0xDFFF range)
• ASCII operations: Char::is_ascii_digit() with boundary cases
• Comparison operators: Equality and ordering for various character types
• Unicode edge cases: Zero-width characters (ZWS, ZWJ, soft hyphen), emoji, CJK scripts, mathematical/currency symbols

Example Coverage

test "Char conversion from Int" {
  // Valid conversions
  inspect((65 : Int).to_char(), content="Some('A')")
  inspect((0x1F600 : Int).to_char(), content="Some('😀')")
  
  // Surrogate range returns None
  inspect((0xD800 : Int).to_char(), content="None")
  inspect((0xDFFF : Int).to_char(), content="None")
}

test "Char utf16_len" {
  inspect('A'.utf16_len(), content="1")      // BMP: single code unit
  inspect('😀'.utf16_len(), content="2")     // Supplementary: surrogate pair
}

Added 12 test blocks (~197 lines), increasing coverage from 28 to 40 test blocks.

Original prompt

write some tests for char

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[coveralls]

Pull Request Test Coverage Report for Build 2055

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 90.844%

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Totals
Change from base Build 2052:	0.0%
Covered Lines:	10160
Relevant Lines:	11184

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💛 - Coveralls