Replace progpib.py stub with a proper installable package

Builds a modern, testable API for the Prologix GPIB-ETHERNET controller
(GPIB-USB structurally supported but untested, no hardware available):
Transport abstraction (Ethernet now, serial later), GpibController for
the full "++" command set, a clean-room netfinder discovery client that
fixes a real chr()/struct-format bug present in vendortools/nfutil.py,
and a CLI (discover/config/info/terminal). vendortools/, pty-gpib-emulator/,
sampledata.txt and the manual PDF are kept untouched as reference material.

Depends on the sibling bits package (editable, not yet pinned -- see its
own commit) for MAC formatting and IEEE-488.2 status-byte bit access.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/test_transport_ethernet.py

@@ -0,0 +1,101 @@
+import time
+
+from progpib.exceptions import TransportClosed, TransportTimeout
+from progpib.transport import EthernetTransport
+
+
+def test_read_line_reassembles_split_recv(mock_server):
+    def handler(conn):
+        conn.sendall(b"hello, wo")
+        time.sleep(0.05)
+        conn.sendall(b"rld\n")
+        time.sleep(0.2)
+
+    server = mock_server(handler)
+    transport = EthernetTransport(server.host, server.port)
+    transport.open()
+    try:
+        line = transport.read_line(timeout=2.0)
+        assert line == b"hello, world"
+    finally:
+        transport.close()
+
+
+def test_read_line_strips_crlf(mock_server):
+    def handler(conn):
+        conn.sendall(b"line one\r\n")
+        time.sleep(0.2)
+
+    server = mock_server(handler)
+    transport = EthernetTransport(server.host, server.port)
+    transport.open()
+    try:
+        assert transport.read_line(timeout=2.0) == b"line one"
+    finally:
+        transport.close()
+
+
+def test_read_line_times_out_with_no_data(mock_server):
+    def handler(conn):
+        time.sleep(0.5)
+
+    server = mock_server(handler)
+    transport = EthernetTransport(server.host, server.port)
+    transport.open()
+    try:
+        try:
+            transport.read_line(timeout=0.1)
+            assert False, "expected TransportTimeout"
+        except TransportTimeout:
+            pass
+    finally:
+        transport.close()
+
+
+def test_read_until_quiet_stops_after_idle_gap(mock_server):
+    def handler(conn):
+        conn.sendall(b"abc")
+        time.sleep(0.05)
+        conn.sendall(b"def")
+        time.sleep(0.5)  # long gap -- read_until_quiet should stop before this
+        conn.sendall(b"ghi")
+
+    server = mock_server(handler)
+    transport = EthernetTransport(server.host, server.port)
+    transport.open()
+    try:
+        data = transport.read_until_quiet(timeout=2.0, idle=0.2)
+        assert data == b"abcdef"
+    finally:
+        transport.close()
+
+
+def test_read_until_quiet_returns_empty_with_no_data(mock_server):
+    def handler(conn):
+        time.sleep(0.3)
+
+    server = mock_server(handler)
+    transport = EthernetTransport(server.host, server.port)
+    transport.open()
+    try:
+        assert transport.read_until_quiet(timeout=0.15) == b""
+    finally:
+        transport.close()
+
+
+def test_recv_raises_closed_on_peer_disconnect(mock_server):
+    def handler(conn):
+        conn.close()
+
+    server = mock_server(handler)
+    transport = EthernetTransport(server.host, server.port)
+    transport.open()
+    try:
+        time.sleep(0.1)
+        try:
+            transport.read_line(timeout=1.0)
+            assert False, "expected TransportClosed"
+        except TransportClosed:
+            pass
+    finally:
+        transport.close()