json_emit.nx

json_emit.nx -- RFC 8259 JSON writer (complement to json.nx parser). Used by: TLS session tickets, API responses, config serialisation, structured logs, Nishi Pages content negotiation (Accept: json). Shape: - Caller allocates an output buffer; we append into it. - A JsonWriter struct tracks position + depth + "needs comma" state so the caller doesn't manage separators manually. - Begin/end array/object and emit_* primitives for scalars. - String escaping per RFC 8259 §7 (\\, \", \/, \b, \f, \n, \r, \t, \uXXXX for < 0x20 + > 0x7E). Handles UTF-8 passthrough for BMP + astral. Invariants: JE1 Output is valid JSON for any sequence of begin/end + emit calls that matches the grammar. Callers can misuse (nest mismatched) and get garbage, but we never emit syntax errors in-pattern. JE2 String emission escapes ALL required characters per RFC 8259 §7; no silent acceptance of control bytes that would produce ambiguous output. JE3 Buffer overruns return a negative error; no silent truncation (malformed JSON tail is worse than a visible error). JE4 Emit is single-pass, no back-patching; total size can be computed up-front from the structure if needed.