refactor: move JsonFormatter into JsonFormatter.cpp

JsonFormatter and BufferedJsonFormatter move verbatim out of
WResp.cpp/WResp.h, which now include JsonFormatter.h.  They don't need
the web server, so on their own they can be unit tested on the PC; the
native env builds the new file.  AsyncJsonResp and JsonSockEvent stay
in WResp.  No behaviour change.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/JsonFormatter.h

@@ -0,0 +1,61 @@
+#ifndef json_formatter_h
+#define json_formatter_h
+#include <Arduino.h>
+
+// Streams JSON into a fixed buffer.  Free of the web server so it can be unit
+// tested on the PC (pio test -e native).
+class JsonFormatter {
+  protected:
+    char *buff;
+    size_t buffSize;
+    bool _headersSent = false;
+    uint8_t _objects = 0;
+    uint8_t _arrays = 0;
+    bool _nocomma = true;
+    char _numbuff[25] = {0};
+    virtual void _safecat(const char *val, bool escape = false);
+    void _appendNumber(const char *name);
+  public:
+    void escapeString(const char *raw, char *escaped);
+    uint32_t calcEscapedLength(const char *raw);
+    void beginObject(const char *name = nullptr);
+    void endObject();
+    void beginArray(const char *name = nullptr);
+    void endArray();
+    void appendElem(const char *name = nullptr);
+
+    void addElem(const char* val);
+    void addElem(float fval);
+    void addElem(int8_t nval);
+    void addElem(uint8_t nval);
+    /*
+    void addElem(int32_t nval);
+    void addElem(int16_t nval);
+    void addElem(uint16_t nval);
+    void addElem(unsigned int nval);
+    */
+    void addElem(int32_t lval);
+    void addElem(uint32_t lval);
+    void addElem(bool bval);
+    
+    void addElem(const char* name, float fval);
+    void addElem(const char* name, int8_t nval);
+    void addElem(const char* name, uint8_t nval);
+    /*
+    void addElem(const char* name, int nval);
+    void addElem(const char* name, int16_t nval);
+    void addElem(const char* name, uint16_t nval);
+    void addElem(const char* name, unsigned int nval);
+    */
+    void addElem(const char* name, int32_t lval);
+    void addElem(const char* name, uint32_t lval);
+    void addElem(const char* name, bool bval);
+    void addElem(const char *name, const char *val);
+    void addElem(const char* name, uint64_t lval);
+};
+class BufferedJsonFormatter : public JsonFormatter {
+  public:
+    void setBuffer(char *b, size_t sz);
+    size_t length() const;
+};
+#endif