feat: add SocketTransport and ach_server.py inbound ACH server

minimateplus/transport.py:
- Add SocketTransport(TcpTransport) — wraps an already-accepted inbound
  socket; connect() is a no-op; everything else inherited from TcpTransport.
  Enables the ACH server to reuse all existing protocol/client code without
  any changes.

bridges/ach_server.py:
- Minimal inbound ACH server — listens on port 12345, accepts call-home
  connections from MiniMate Plus units, runs the full BW protocol:
  startup handshake → get_device_info → get_events(full_waveform=True)
- Saves device_info.json + events.json + raw_rx_<ts>.bin + session log
  per connection to bridges/captures/ach_inbound_<ts>/
- raw_rx.bin is byte-compatible with existing Analyzer tooling
- Taps transport.read() to capture raw S3 bytes alongside parsed output
- Each connection runs in its own daemon thread
- Clearly distinguishes push vs pull protocol in the startup log

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

minimateplus/transport.py

@@ -418,3 +418,39 @@ class TcpTransport(BaseTransport):
     def __repr__(self) -> str:
         state = "connected" if self.is_connected else "disconnected"
         return f"TcpTransport({self.host!r}, port={self.port}, {state})"
+
+
+# ── Inbound / accepted-socket transport ───────────────────────────────────────
+
+class SocketTransport(TcpTransport):
+    """
+    Like TcpTransport but wraps an already-accepted inbound socket.
+
+    Used by the ACH inbound server (bridges/ach_server.py) — the device dials
+    IN to us, so by the time we create this transport the socket is already live.
+    connect() is a no-op; everything else (read, write, read_until_idle, …) is
+    inherited unchanged from TcpTransport.
+
+    Args:
+        sock: An already-connected socket.socket returned by server_socket.accept().
+        peer: Human-readable peer label for repr / logging (e.g. "203.0.113.5:54321").
+    """
+
+    def __init__(self, sock: socket.socket, peer: str = "inbound") -> None:
+        # Bypass TcpTransport.__init__ — we already have a live socket.
+        self.host            = peer
+        self.port            = 0
+        self.connect_timeout = 0.0
+        self._sock           = sock
+        sock.settimeout(self._RECV_TIMEOUT)
+
+    def connect(self) -> None:
+        """No-op — socket was already accepted inbound."""
+        pass  # Already have a live socket; nothing to open.
+
+    @property
+    def is_connected(self) -> bool:
+        return self._sock is not None
+
+    def __repr__(self) -> str:
+        return f"SocketTransport(peer={self.host!r})"