cJSON JsonWriter backend, proven by porting the profiles exporter #109

Parent

Part of #102

What to build

A cJSON-backed JsonWriter for ESP-IDF, proven by porting the existing profiles exporter onto it and verifying on hardware. This is the validation of the upstream interface: a second, independent consumer that shakes out vocabulary gaps before the design is proposed upstream.

Target: this repository, branch json-writer-cjson.

cJSON is a document library, so the backend maintains a node stack translating begin/end tokens into node construction. This does not realise a streaming memory benefit; it buys the option at no extra cost, and cJSON is chosen deliberately to minimise hand-maintained code and to reuse the correct string escaping and number formatting it already provides. It is already a declared dependency of this component.

The profiles exporter currently hand-rolls cJSON calls. Porting it leaves the component with one JSON emission path instead of two.

Acceptance criteria

• A cJSON-backed JsonWriter implements the full token vocabulary
• It compiles with exceptions and RTTI disabled
• Failures set the sticky flag and are logged, never thrown
• The profiles exporter emits exclusively through the interface
• Profile payloads are byte-equivalent to those produced before the port
• The existing profiles end-to-end test passes against the headless stack with the real device
• Any vocabulary gap found during the port is reported back so the upstream interface can be corrected before review

Blocked by

• Make nlohmann-json excludable from the build #107