Raise host frame payload limit so multi-client UART replies fit.

Use a uint16 LE length and 1024-byte max payload so CLIENT_INFO and BATTERY_STATUS no longer fail encoding past ~10 connected pods.

Co-authored-by: Cursor <cursoragent@cursor.com>

main/uart.h

@@ -22,13 +22,18 @@
 #define UART_DRIVER_TX_BUF_SIZE 4096
 #define START_MARKER 0xAA
 #define STOP_MARKER 0xCC
-#define MAX_BUF_SIZE 252
-#define MAX_PAYLOAD_SIZE                                                       \
-  MAX_BUF_SIZE - 4 // Buffer overhead, Start, Len, CRC, End
+/** Frame: AA | len_lo | len_hi | payload[len] | xor | CC */
+#define UART_FRAME_OVERHEAD 5
+/**
+ * Max application payload (type byte + protobuf). Sized for CLIENT_REGISTRY_MAX
+ * aggregations (CLIENT_INFO / BATTERY_STATUS with 16+ samples).
+ */
+#define MAX_PAYLOAD_SIZE 1024
 
 typedef enum {
   STATE_START,
-  STATE_LEN,
+  STATE_LEN_LO,
+  STATE_LEN_HI,
   STATE_DATA,
   STATE_CHECKSUM,
   STATE_STOP
@@ -36,8 +41,8 @@ typedef enum {
 
 typedef struct {
   uint8_t payload[MAX_PAYLOAD_SIZE];
-  uint8_t len;
-  uint8_t index;
+  uint16_t len;
+  uint16_t index;
   uint8_t checksum;
   parser_state_t state;
 } uart_packet_t;