feat(scada): add calibrated monitoring and incident recovery

tests/fixtures/server_handoff_app.py

@@ -0,0 +1,60 @@
+"""Spawn-safe HTTP/SSE fixture, controlled only through a test-owned directory."""
+import asyncio
+from contextlib import asynccontextmanager
+import json
+import os
+from pathlib import Path
+import time
+
+from fastapi import FastAPI
+from starlette.responses import StreamingResponse
+
+
+@asynccontextmanager
+async def lifespan(app: FastAPI):
+    directory = Path(os.environ["SCADA_HANDOFF_FIXTURE_DIR"])
+    pid = os.getpid()
+    app.state.directory = directory
+    app.state.version = (directory / "version").read_text().strip()
+    mode = (directory / "startup-mode").read_text().strip()
+    (directory / f"started-{pid}").write_text(app.state.version)
+    if mode == "fail":
+        raise RuntimeError("test candidate startup failed")
+    if mode == "delay":
+        await asyncio.sleep(0.3)
+    while mode == "wait" and not (directory / "allow-startup").exists():
+        await asyncio.sleep(0.02)
+    try:
+        yield
+    finally:
+        (directory / f"stopped-{pid}").write_text(app.state.version)
+
+
+app = FastAPI(lifespan=lifespan)
+
+
+async def readiness_check() -> bool:
+    directory = Path(os.environ["SCADA_HANDOFF_FIXTURE_DIR"])
+    ready = (directory / "readiness").read_text().strip() == "true"
+    (directory / f"checked-{os.getpid()}").write_text(str(ready).lower())
+    await asyncio.sleep(.05)
+    return ready
+
+
+@app.get("/health")
+async def health():
+    return {"pid": os.getpid(), "version": app.state.version}
+
+
+@app.get("/events")
+async def events():
+    async def stream():
+        directory = app.state.directory
+        pid = os.getpid()
+        while not (directory / f"release-{pid}").exists() and not (directory / "release-all").exists():
+            payload = {"pid": pid, "version": app.state.version, "time_ns": time.time_ns()}
+            yield f"data: {json.dumps(payload)}\n\n"
+            await asyncio.sleep(0.025)
+        yield f"event: complete\ndata: {json.dumps({'pid': pid})}\n\n"
+
+    return StreamingResponse(stream(), media_type="text/event-stream")