docs: add Troubleshooting section + serial_monitor.py diagnostic tool

- README: note NVS may be cleared by firmware uploads (requires re-running
  flash_device.py); new Troubleshooting table covering the fast-blink fatal
  state, captive-portal fallback, and no-counts cases.
- tools/serial_monitor.py: ESP32 RTS/DTR reset + serial capture with
  per-line elapsed-time prefix. Used to distinguish "unprovisioned" vs
  "WiFi failed" boot states (fast-blink LED alone is ambiguous).
- README project-tree updated to include lib/cv, docs/server-prompt-…,
  and the new tool.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tools/serial_monitor.py

@@ -0,0 +1,56 @@
+#!/usr/bin/env python3
+# Serial monitor for ESP32. Optionally pulses RTS/DTR to reset the device
+# so we capture boot output. Prefixes each line with elapsed seconds.
+import serial, sys, time, argparse
+
+def main():
+    ap = argparse.ArgumentParser()
+    ap.add_argument("--port",    default="/dev/ttyUSB0")
+    ap.add_argument("--baud",    type=int, default=115200)
+    ap.add_argument("--seconds", type=int, default=20)
+    ap.add_argument("--reset",   action="store_true",
+                    help="Pulse RTS/DTR to reset the ESP32 before reading")
+    ap.add_argument("--timestamp", action="store_true",
+                    help="Prefix each line with elapsed seconds since boot")
+    args = ap.parse_args()
+
+    try:
+        s = serial.Serial(args.port, args.baud, timeout=0.2,
+                          rtscts=False, dsrdtr=False)
+    except Exception as e:
+        print(f"[open-fail] {e}", flush=True)
+        sys.exit(2)
+
+    if args.reset:
+        s.setDTR(False)
+        s.setRTS(True)
+        time.sleep(0.1)
+        s.setRTS(False)
+        s.reset_input_buffer()
+
+    t0 = time.time()
+    end = t0 + args.seconds
+    buf = b""
+    while time.time() < end:
+        chunk = s.read(512)
+        if chunk:
+            buf += chunk
+            while b"\n" in buf:
+                line, buf = buf.split(b"\n", 1)
+                text = line.decode("utf-8", errors="replace").rstrip("\r")
+                if args.timestamp:
+                    sys.stdout.write(f"[{time.time()-t0:5.1f}s] {text}\n")
+                else:
+                    sys.stdout.write(text + "\n")
+                sys.stdout.flush()
+    if buf:
+        text = buf.decode("utf-8", errors="replace")
+        if args.timestamp:
+            sys.stdout.write(f"[{time.time()-t0:5.1f}s] {text}\n")
+        else:
+            sys.stdout.write(text)
+        sys.stdout.flush()
+    s.close()
+
+if __name__ == "__main__":
+    main()