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/secsgem_client/_values.py

@@ -0,0 +1,46 @@
+"""Plain Python values <-> the wire's Value message.
+
+The daemon owns all SECS-II knowledge (it converts to each variable's
+declared wire format); this layer only maps Python types onto the Value
+oneof. Order matters in to_value: bool is checked before int because
+isinstance(True, int) is True in Python.
+"""
+
+from __future__ import annotations
+
+from ._proto import equipment_pb2 as pb
+
+
+def to_value(v) -> pb.Value:
+    if isinstance(v, pb.Value):
+        return v
+    if isinstance(v, bool):
+        return pb.Value(boolean=v)
+    if isinstance(v, int):
+        return pb.Value(integer=v)
+    if isinstance(v, float):
+        return pb.Value(real=v)
+    if isinstance(v, str):
+        return pb.Value(text=v)
+    if isinstance(v, (bytes, bytearray)):
+        return pb.Value(binary=bytes(v))
+    if isinstance(v, (list, tuple)):
+        return pb.Value(list=pb.List(items=[to_value(e) for e in v]))
+    raise TypeError(f"cannot convert {type(v).__name__} to a SECS value")
+
+
+def from_value(v: pb.Value):
+    kind = v.WhichOneof("kind")
+    if kind == "text":
+        return v.text
+    if kind == "integer":
+        return v.integer
+    if kind == "real":
+        return v.real
+    if kind == "boolean":
+        return v.boolean
+    if kind == "binary":
+        return v.binary
+    if kind == "list":
+        return [from_value(e) for e in v.list.items]
+    return None  # unset