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>

README.md

@@ -1,3 +1,54 @@
 # progpib
 
-Prologix GPIB python tools.
+A modern Python 3 API for the [Prologix](https://prologix.biz/) GPIB-ETHERNET
+controller. GPIB-USB is supported behind the same interface but is
+**untested** -- no GPIB-USB hardware was available while building this
+package (see `progpib/transport/serial.py`).
+
+## Install (development)
+
+This package depends on [`bits`](https://git.groesz.org/Groesz.org/bits)
+(distribution name `binary-bits`), currently used as a local editable
+checkout rather than a pinned PyPI release -- install it first:
+
+```sh
+python -m venv .venv
+.venv/bin/pip install -e ../bits
+.venv/bin/pip install -e .[dev]
+```
+
+## Usage
+
+```python
+from progpib import discover, GpibController
+
+# Find controllers on the LAN (broadcasts on every local interface):
+for device in discover():
+    print(device.mac_str, device.ip_address, device.name)
+
+# Talk to one:
+with GpibController.ethernet("192.168.1.50") as gpib:
+    print(gpib.ver())
+    gpib.addr = 5  # address the instrument at GPIB primary address 5
+    gpib.eos = 3   # no GPIB terminator appended
+    gpib.eoi = 1
+    response = gpib.query("*IDN?")
+    print(response.decode())
+```
+
+## CLI
+
+```sh
+progpib discover                      # find controllers on the LAN
+progpib info <ip>                     # controller version/mode/config
+progpib config --mac AA:BB:CC:DD:EE:FF --dhcp
+progpib config --mac AA:BB:CC:DD:EE:FF --static 192.168.1.50 255.255.255.0 192.168.1.1
+progpib terminal <ip> --addr 5        # interactive pass-through terminal
+```
+
+## Reference material
+
+`vendortools/`, `pty-gpib-emulator/`, `sampledata.txt` and
+`PrologixGpibEthernetManual.pdf` are kept as historical/reference material
+(original vendor tooling, a separate PTY-based emulator side project, and a
+captured protocol session) -- nothing in `progpib/` imports from them.