[Shy7777]

A valid JSON string containing an escaped U+0000 character can be parsed successfully, but writing it back currently emits a literal NUL. Other control characters without a short escape are also written verbatim, producing invalid JSON and potentially truncating the output for callers that treat it as a C string.

This escapes the remaining U+0000–U+001F characters as \u00hh in both the minified and pretty writers, including object keys. The size calculation accounts for all six bytes. Existing short escapes and UTF-8 bytes are preserved.

The new regressions check all 32 control characters, embedded NUL followed by more text, object keys and values, output sizes, and UTF-8. They fail against the original writer and pass with this change. The full CMake/CTest suite passes locally with Apple Clang 17 across all nine C/C++ test variants, including the AddressSanitizer, UndefinedBehaviorSanitizer, and ThreadSanitizer targets.

Related to #103. This addresses serialization only; parser acceptance rules are unchanged. The output follows the escaping requirement in RFC 8259, section 7.

[Copilot]

🟢 Approval recommended

No unresolved issues were identified, and all reviewers assessed it as ready.

Pull request overview

This PR updates JSON writers to escape all control characters (U+0000–U+001F) correctly while preserving short escapes and UTF-8 output.

Changes:

• Updates string size calculations.
• Escapes controls in values and object keys.
• Adds regression coverage for minified and pretty writers.

File summaries

File	Description
test/test_shared.h	Adds comprehensive serialization regression tests.
json.h	Implements control-character escaping and matching size calculations.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Lite

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[sheredom]

Thanks for your change!