feat: socket protocol 2 core on ws://host/ws

The /ws endpoint runs next to the old socket on port 8080.  The
handshake checks the token under security, the device sends hello,
then a sync of the schema (unless the client has it), header, rooms,
shades, groups, repeaters and status, paced by the client's next.
Events go out with the old ones: shadeState or the whole shade on a
config change, groups, rooms, removals, orders, repeaters,
shadeCommand, memory, wifi, ethernet, and the frames and
frequencyScan feeds.

The table of types, fields, errors and feeds is defined once and is
sent as the schema.  The writer, item writers, sync and client frame
parser are pure and natively tested.

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

src/JsonFormatter.h

@@ -2,6 +2,15 @@
 #define json_formatter_h
 #include <Arduino.h>
 
+// Where a formatter's output stands, to go back to when what follows doesn't
+// fit (JsonFormatter::mark()).
+struct json_mark_t {
+  size_t len;
+  uint8_t objects;
+  uint8_t arrays;
+  bool nocomma;
+};
+
 // 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 {
@@ -16,6 +25,8 @@ class JsonFormatter {
     bool _nocomma = true;
     // Something didn't fit and was left out so the JSON is incomplete.
     bool _overflow = false;
+    // Whether running out of room is logged.
+    bool _logOverflow = true;
     char _numbuff[25] = {0};
     virtual void _safecat(const char *val, bool escape = false);
     // Appends val at buff + _len.  vlen is what it takes, including the quotes
@@ -24,6 +35,12 @@ class JsonFormatter {
     void _appendNumber(const char *name);
   public:
     bool overflowed() const { return _overflow; }
+    json_mark_t mark() const { return {_len, _objects, _arrays, _nocomma}; }
+    // Drops everything written since m, an overflow included.
+    void rollback(const json_mark_t &m);
+    // A writer that tries an item and rolls back when it doesn't fit turns the
+    // overflow log off, as running out of room is expected there.
+    void logOverflow(bool on) { _logOverflow = on; }
     // Drops whatever is written until a buffer is set again.  Nothing is
     // written anywhere, so tasks can share one discarding formatter.
     void discard();