feat: rewrite SniffSession with live streaming and REPL auto-load

Replace lightweight async SniffSession with persistent synchronous
session using raw pyserial (mirroring SimSession pattern). Background
reader thread decodes and prints trace frames live as they arrive.

API:
  session = SniffSession.open()
  session.start_15693()          # live streaming (default)
  session.start_15693(live=False) # legacy batch mode
  session.stop()                 # or button press to pause/resume
  session.entries                # all captured frames
  session.clear()

.pythonstartup.py auto-creates session in sniff mode.

.pythonstartup.py

@@ -95,6 +95,22 @@ for module_name, attr_name in imports:
 if loaded:
     print(f"[pm3py:{_MODE}] Loaded: {', '.join(loaded.keys())}")
 
+    if _MODE == "core":
+        try:
+            pm3 = Proxmark3.sync()
+            print()
+            print("Synchronous connection established")
+            print()
+        except Exception as e:
+            print(f"  (Proxmark3 connection failed: {e})")
+
+    if _MODE == "sniff":
+        try:
+            session = SniffSession.open()
+            print("Sniff session ready -- session.start_15693()")
+        except Exception as e:
+            print(f"  (Sniff session failed: {e})")
+
 if failed:
     print(f"\n[pm3py:{_MODE}] Failed:")
     for name, (mod, err) in failed.items():