Complete JSON text support with core Unicode module (no literal-registered methods)

[Copilot]

This PR completes the JSON library in Coderive and resolves the Unicode gap by moving Unicode escape logic into reusable stdlib code, not literal-registered runtime methods. It also aligns newly introduced API/labels with text terminology.

• Problem addressed

  • JSON \uXXXX handling was incomplete/inconsistent and coupled to JSON internals.
  • Unicode behavior needed to be reusable across Coderive, not implemented as literal-registered methods.

• JSON module: Unicode-correct parsing + serialization

  • Updated std/json/Json.cod to canonicalize and validate Unicode escapes in JSON text values.
  • Enforces surrogate-pair correctness (high must be followed by valid low; rejects invalid lone surrogates).
  • Preserves canonical escaped form (\uXXXX, including surrogate pairs) through parse/serialize flows.

• New core Unicode standard library

  • Added std/unicode/Unicode.cod with reusable helpers:
    • normalizeEscaped(text) :: text
    • isValidEscaped(text) :: bool
    • hex/surrogate utility functions used by JSON
  • This makes Unicode escape handling a general Coderive capability rather than JSON-only behavior.

• Removed runtime literal-method dependency

  • Removed newly added literal-registered Unicode methods from LiteralRegistry.
  • JSON and Unicode behavior now rely on Coderive stdlib code paths.

• API/terminology alignment (text over string)

  • JSON API surface updated to text-first naming:
    • isText, asText, makeText, text(...)
    • serialize(...), serializePretty(...), serializeValue(...)
    • parseText(...), escapeText(...)
  • Updated JSON tests accordingly.

• Test coverage additions/updates

  • Added test/unicode/UnicodeStandardLibraryComprehensive.cod.
  • Updated test/json/JsonStandardLibraryComprehensive.cod for text naming and Unicode canonicalization expectations.

use {json, unicode}

escaped := Unicode.normalizeEscaped("\\ud83d\\ude00")
ok := Unicode.isValidEscaped(escaped)

v := Json.parse("\"\\uD83D\\uDE00\"")
out(v.asText())            // \uD83D\uDE00
out(Json.serialize(v))     // "\uD83D\uDE00"