feat(client): the Python client — a GEM tool in plain Python (Phase C)

clients/python: pip-installable "secsgem-client", pure Python (stubs
pre-generated from equipment.proto, import made package-relative; no
compiled extension, no SEMI knowledge, no C++ toolchain). The API the whole
effort aimed at:

    eq = Equipment("localhost:50051")
    eq.set(ChamberPressure=2.5); eq["WaferCounter"] = 7
    eq.fire("ProcessStarted", ChamberPressure=2.75)
    eq.alarm("chiller_temp_high"); eq.clear("chiller_temp_high")
    @eq.on("START")
    def start(cmd): ...           # auto-CompleteCommand after return
    eq.listen(background=True)
    eq.control_state; eq.request_control_state("HOST_OFFLINE"); eq.health()

Errors raise SecsGemError carrying the daemon's message ("no variable named
..."). bool checked before int in conversion (isinstance(True, int)).
examples/mini_tool.py is a complete GEM tool in ~25 lines.

PROOF — interop/pyclient_interop.py drives the PUBLISHED package (not raw
stubs) against a live secs_gemd with secsgem-py as the fab host: 13 checks
all green on first run — set/get round-trips, item syntax, SecsGemError on
unknown names, control state, health, fire->S6F11 on the host's wire,
alarm/clear->S5F1 with correct set bit, the full command loop (host S2F41 ->
HCACK=4 -> @eq.on handler -> completion event back at the host), operator
offline. Conversion layer unit-tested standalone; both wired into
tools/run_interop.sh as the pyclient step.

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

clients/python/tests/test_values.py

@@ -0,0 +1,35 @@
+#!/usr/bin/env python3
+"""Conversion round-trips for the Value layer. Plain asserts — run directly."""
+import sys
+
+from secsgem_client._proto import equipment_pb2 as pb
+from secsgem_client._values import from_value, to_value
+
+
+def roundtrip(v):
+    return from_value(to_value(v))
+
+
+def main() -> int:
+    assert roundtrip(2.5) == 2.5
+    assert roundtrip(7) == 7
+    assert roundtrip(-3) == -3
+    assert roundtrip(True) is True          # bool BEFORE int: must stay boolean
+    assert roundtrip(False) is False
+    assert to_value(True).WhichOneof("kind") == "boolean"
+    assert to_value(1).WhichOneof("kind") == "integer"
+    assert roundtrip("wafer-17") == "wafer-17"
+    assert roundtrip(b"\x01\x02") == b"\x01\x02"
+    assert roundtrip([1, 2.5, "x", [True]]) == [1, 2.5, "x", [True]]
+    assert from_value(pb.Value()) is None   # unset oneof
+    try:
+        to_value(object())
+        raise SystemExit("expected TypeError for unconvertible type")
+    except TypeError:
+        pass
+    print("values: all conversion checks passed")
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())