Add production native JSON utility and replace EscapeJsonString copies

[doublemover]

Parent inventory: #8108

Objective

Create a production-only native JSON utility and replace duplicated EscapeJsonString implementations without adding a third-party JSON dependency yet.

Shared Module Specification

• Add native/objc3c/src/io/objc3_json.h and native/objc3c/src/io/objc3_json.cpp, or an equivalent dependency-correct production location.
• Expose objc3::io::EscapeJsonString(std::string_view) and WriteJsonString(std::ostream&, std::string_view).
• Optionally expose tiny object/field emission helpers if doing so removes malformed-JSON risk without creating a framework.
• Keep this production helper separate from test-only probe JSON helpers.
• Do not add a third-party JSON dependency unless this issue records a deliberate decision and build/distribution impact.

Migration Checklist

• Add native tests or focused acceptance coverage for JSON escaping before replacing all copies.
• Replace each duplicate EscapeJsonString implementation.
• Update native CMake/source lists for the new module if needed.
• Preserve artifact schemas and emitted JSON text except for intentional validated escaping fixes.

Complete Replacement Inventory

• EscapeJsonString - 5 definition(s)/local occurrence(s) to replace or explicitly document as intentionally local if semantics differ.
  • native/objc3c/src/io/objc3_diagnostics_artifacts.cpp:13
  • native/objc3c/src/io/objc3_process.cpp:334
  • native/objc3c/src/libobjc3c_frontend/frontend_anchor.cpp:183
  • native/objc3c/src/pipeline/objc3_frontend_artifacts.cpp:317
  • native/objc3c/src/tools/objc3c_frontend_c_api_runner.cpp:185

Performance And Reliability Requirements

• Keep helper code small, dependency-light, and fast enough for normal development feedback.
• Add focused tests for edge cases before broad migration.
• Preserve existing output schemas and public behavior unless an intentional bug fix is validated.
• Keep production helpers separated from test-only helpers.
• Do not use tmp/ as durable source-of-truth; generated temporary files are allowed only as transient command inputs.

Fast Tests

• Run native JSON helper tests or focused C++ smoke tests.
• Run native build.
• Run frontend/reporting checks that consume JSON artifacts.
• Run runtime acceptance after all replacements.
• Run git diff --check.

End-To-End Completion Criteria

• Only one production native JSON escaping implementation remains.
• All five previous call sites use the shared helper.
• No production source includes test-only probe JSON support.

[doublemover]

Completed in commit ff2ac040a.\n\nWhat changed:\n- Added production native JSON helper module
ative/objc3c/src/io/objc3_json.{h,cpp} with objc3::io::EscapeJsonString(std::string_view) and objc3::io::WriteJsonString(std::ostream&, std::string_view).\n- Replaced the five local EscapeJsonString implementations in objc3_diagnostics_artifacts.cpp, objc3_process.cpp, rontend_anchor.cpp, objc3_frontend_artifacts.cpp, and objc3c_frontend_c_api_runner.cpp with the shared production helper.\n- Wired objc3c_json into the CMake native target graph and the PowerShell native source graph/invalidation inputs.\n- Added scripts/check_objc3c_native_json_helper.py as a fast compile/run smoke test for quote, slash, control, short escape, and quoted string writer behavior.\n\nValidation run:\n- python scripts/check_objc3c_native_json_helper.py passed.\n- clang++ -std=c++20 -I native/objc3c/src -fsyntax-only native/objc3c/src/io/objc3_json.cpp passed.\n- python scripts/check_objc3c_dependency_boundaries.py --strict passed.\n- pwsh -NoProfile -File scripts/build_objc3c_native.ps1 -ExecutionMode binaries-only passed.\n- python scripts/check_objc3c_runtime_acceptance.py --suite fast passed.\n- python scripts/build_objc3c_editor_tooling_surface.py passed.\n- python scripts/objc3c_public_workflow_runner.py check-public-command-contract passed.\n- git diff --check / staged diff check passed.\n\n
g confirms only the production helper declaration/definition remain for EscapeJsonString; migrated call sites now use the shared helper.