Add JSON migration documentation and backend abstraction proposal

[Easton97-Jens]

Motivation

• The codebase currently relies heavily on YAJL in multiple roles (streaming/SAX parser, writer/serializer and DOM/test utilities) and needs a migration path to replace YAJL without breaking behavior.
• A clear evaluation of alternative JSON libraries and an integration strategy are required to plan a safe, phased migration.
• The repository build systems and tests must be considered when introducing replaceable backends and packaging options.

Description

• Add a comprehensive docs/json-migration/ handbook covering an overview, per-library evaluations, architecture options, build-system integration, recommendation matrix and a phased migration plan.
• Provide per-library assessments (RapidJSON, nlohmann/json, json-c, jansson, cJSON, jsoncpp, jsoncons, simdjson, yyjson, glaze) in both German and English under docs/json-migration/ and docs/json-migration/en/.
• Propose an internal abstraction (src/json_backend/ sketch) including IJsonStreamParser, IJsonWriter, capability flags and an approach to support separate parser/writer/DOM backends.
• Document Autotools and CMake integration suggestions, CI matrix changes, pkg-config considerations and a prioritized PoC sequence (YAJL adapter → json-c → jansson/yyjson → optional C++ backends).

Testing

• No automated tests were run for this change because the submission is documentation-only and does not modify runtime code or tests.

---

Codex Task