Integrate Unicode escape handling into lexer/parser and remove unicode std dependency from JSON

[Copilot]

This change moves full \uXXXX escape handling into compiler string parsing and eliminates the JSON library’s dependency on the Unicode std module. JSON now handles Unicode escape parsing/normalization through local helpers while the obsolete Unicode std package is removed.

• Lexer/parser Unicode handling

  • Added full Unicode escape decoding for string literals in Java lexer/parser paths.
  • Supports surrogate-pair validation (high + required low surrogate) and rejects malformed/lone surrogate escapes.
  • Keeps escape semantics centralized in language parsing rather than stdlib post-processing.

• JSON stdlib decoupling

  • Removed use {unicode} from json stdlib.
  • Replaced all Unicode.* calls with local JSON helpers (isDigit, isHexDigit, hexValue, hex4, isHighSurrogate, isLowSurrogate).
  • Preserves JSON \u escape normalization behavior without external stdlib coupling.

• Stdlib cleanup

  • Removed std/unicode/Unicode.cod.
  • Removed the dedicated Unicode stdlib comprehensive test.
  • Added ignore entries for generated demo runtime artifacts under src/main/cod/demo/src/bin/ and src/main/cod/demo/src/idx/.

Example of the new parser-side behavior:

// now decoded by lexer/parser (with surrogate validation), not stdlib
if (escaped == 'u') {
    UnicodeEscapeResult unicodeResult = decodeUnicodeEscape();
    escapedStr = unicodeResult.text;
}