perf(serial): improve throughput test stepping and failure handling

- Change size stepping from 2x to 1.4x with even rounding for finer
  granularity (16→22→30→42→58→80→112→156→218→304→424→592→828→...→3174)
- When minimum size (16 bytes) fails, report as error instead of
  recommending 16 bytes as chunk size
- Set recommended_chunk_size to 0 on total failure

Author: FASTSHIFT

Tools/WebServer/core/serial_protocol.py

@@ -630,7 +630,7 @@ def test_serial_throughput(
                     break
 
                 results["tests"].append(test_result)
-                test_size *= 2
+                test_size = max(test_size + 2, int(test_size * 1.4) // 2 * 2)
 
             results["max_working_size"] = max_working
             if max_working > 0:
@@ -640,7 +640,12 @@ def test_serial_throughput(
                 if results["recommended_chunk_size"] < 16:
                     results["recommended_chunk_size"] = max_working
             else:
-                results["recommended_chunk_size"] = 16
+                results["success"] = False
+                results["error"] = (
+                    "Serial communication failed at minimum size "
+                    f"({start_size} bytes). Check connection and try again."
+                )
+                results["recommended_chunk_size"] = 0
 
         except Exception as e:
             results["success"] = False

Tools/WebServer/static/js/features/fpb.js

@@ -45,7 +45,7 @@ async function fpbTestSerial() {
     return;
   }
 
-  log.info('Starting serial throughput test (x2 stepping)...');
+  log.info('Starting serial throughput test (x1.4 stepping)...');
 
   try {
     const res = await fetch('/api/fpb/test-serial', {