Add support for NI GPIB-ENET/100 bridge.

[bytewarrior]

Summary

Adds a pure-Python session driver for the NI GPIB-ENET/100 Ethernet-to-GPIB bridge, modeled after the existing Prologix support. Once the companion pyvisa changes are merged, one can

rm = pyvisa.ResourceManager("@py")
rm.open_resource("NI-ENET100-TCPIP0::192.0.2.42::INTFC")
inst = rm.open_resource("GPIB0::16::INSTR")  # dispatched through the bridge
print(inst.query("*IDN?"))

with no additional dependencies (stdlib socket / struct / threading only).

Depends on

pyvisa#980 – Add InterfaceType.ni_enet100_tcpip, NIEnet100TCPIPIntfc rname class and Resource subclass.

Until that PR is merged, from pyvisa_py import nienet100 raises ImportError, which highlevel.py swallows at debug level (mirrors the vicp pattern). All NIENET100 tests skip cleanly in that case, so this PR remains importable today; it just stays inert.

What's added

• Wire protocol (pyvisa_py/protocols/nienet100.py)
• Discovery (pyvisa_py/protocols/nienet100_discovery.py) — UDP broadcast on the local broadcast domain.
• Session layer (pyvisa_py/nienet100.py) — NIEnet100TCPIPIntfcSession for NI-ENET100-TCPIP::INTFC, NIEnet100InstrSession for GPIB::INSTR resources that route through a registered bridge.
• Dispatch hook — chains in front of gpib.py's GPIBSessionDispatch and falls back to it for non-NIENET100 boards. Coexists with Prologix: load order is prologix → gpib → nienet100, dispatch order at open_resource is NIENET100 → Prologix → linux-gpib. Users wire each bridge to a distinct board number.
• Module wiring in pyvisa_py/highlevel.py (one new try/except block).
• Tests — offline unit tests for the wire layer, discovery, and SRQ verbs; assisted (hardware-gated) tests for the wire layer and full pyvisa-stack session layer.

Testing

Offline (always runs, no hardware needed):

• pyvisa_py/testsuite/test_nienet100.py (~25 tests) — wire framing, status header parsing, open/close sequences, SRQ verbs.
• pyvisa_py/testsuite/test_nienet100_discovery.py — discovery frame parsers + UDP loop with mocked sockets.

Hardware-gated (pyvisa_py/testsuite/nienet100_assisted_tests/) — opt-in via env vars:

PYVISA_TEST_NIENET100_HOST  # IP or hostname of the bridge
PYVISA_TEST_GPIB_PAD        # primary address of an instrument on the bus
PYVISA_TEST_GPIB_SAD        # (optional) secondary address
PYVISA_TEST_IDN_VENDOR      # (optional) substring expected in *IDN?
PYVISA_TEST_GPIB_TERM       # (optional) write/read termination — defaults to \n

Without PYVISA_TEST_NIENET100_HOST set, every assisted test skips cleanly. Verified locally against a real GPIB-ENET/100.

Out of scope / known limitations

These are documented in the code and listed here so reviewers don't have to chase them:

• Six of the seven gpib_control_ren modes return error_nonsupported_operation — only VI_GPIB_REN_DEASSERT_GTL (which is by far the common case) is wired. The other six need an ibsre verb whose wire frame is not yet reverse-engineered. Prologix exposes none of these modes, so this is already a strict improvement.
• No SRQ event delivery to pyvisa's event model — ibwait is implemented at the wire layer but the bridge to pyvisa's enable_event/wait_on_event machinery is not.
• ibrsp with cnt != 1 raises NIEnet100ProtocolError. The wire spec lets the bridge cram multi-byte STB blocks into a single response; in practice every device we've seen sends a single STB byte. Will relax when an instrument shows otherwise.
• Multi-chunk ibrd data fragmentation — the parser handles it (accumulates into payload across data chunks until END or final-status), but has not been validated against very large device responses.
• *IDN? instrument tests assume \n termination because pyvisa's \r\n default chokes several older GPIB instruments. The PYVISA_TEST_GPIB_TERM env var overrides.

The bridge itself has two intrinsic quirks worth noting for users:

• Built-in timeout minimum ~3 s. Per-call tmo_ms in ibrd does not lower this floor — short pyvisa timeouts (e.g. inst.timeout = 200) surface as the configured error_timeout but after ~3 s wall-clock, not 200 ms. The socket-level timeout ceiling is sized to accommodate.
• Limited concurrent sessions. Rapid open/close cycles can put the bridge into a state where it stops accepting new TCP connections for ~60 s. Practical workaround: serialize bridge sessions, or wait/power-cycle.

Reviewer guidance

The commit history is structured for sequential review — each commit is self-contained and the rough phases are:

1. Wire-protocol primitives (chunks, status headers, command framing).
2. Open / close sequences and Device-I/O verbs.
3. Session-layer integration into pyvisa-py (pyvisa_py/nienet100.py, dispatch hook, module loader).
4. SRQ verbs and lazy wait/control socket lifecycle.
5. Discovery (frame primitives, UDP loop, list_resources wiring).
6. Offline tests.
7. Hardware-gated tests.
8. Wire-level fixes uncovered by hardware testing (ibrsp combined chunk, ibrd no-END path, IbcTMO rejection, status-header chunk wrapping).
9. Session-layer fixes uncovered by full-stack hardware testing (resource-string mapping, board-key type, timeout handling, test robustness).

The protocol was reverse-engineered from wire captures and the gpibhlpr.dll decompilate; key wire-layer details (status-header chunking, ibrsp combined-chunk format, ibrd two-chunk model, tmo_ms semantics) are documented inline at their use sites.

• Executed ruff with no errors
• The change is fully covered by automated unit tests

still open, will continue after initial feedback:

• Documented in docs/ as appropriate
• Added an entry to the CHANGES file