feat(1.2): prefork supervisor primitive + tests (C, CoW gate passed)

CoW measurement on CPython 3.14: forked idle child keeps ~71MB shared,
dirties ~1MB private; working child ~26MB. PEP 683 immortal objects keep
code/module pages clean so gc.freeze() is unnecessary (freeze==nofreeze).

prefork.run_fleet: master imports the base floor once, forks one child
per worker (own process/GIL, CoW-shared floor), reaps + restarts with
backoff, graceful SIGTERM->SIGKILL shutdown. Not yet wired to a command
(that lands when 1.2 picks the target worker set).

tests/prefork_driver.py

@@ -0,0 +1,55 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+"""Standalone driver for the prefork supervisor — runnable directly OR via
+tests/test_prefork.py (which execs it in a subprocess so no fork happens inside
+the pytest/xdist worker).
+
+  python tests/prefork_driver.py <out_dir>
+
+Forks two fake workers under decnet.prefork.run_fleet:
+  * "tick"    — append a line every 0.2s forever (proves a worker runs & stays up)
+  * "crasher" — write a marker then exit(1) (proves restart-on-crash)
+Runs for ~2s via stop_after, then shuts the fleet down. Writes results into
+<out_dir>; the caller asserts on them.
+"""
+from __future__ import annotations
+
+import os
+import sys
+import time
+
+# Running this file as a script puts its own dir (tests/) on sys.path[0], which
+# shadows the stdlib `logging` via tests/logging/. Drop it before importing
+# decnet (still importable — it's installed in the venv).
+if sys.path and os.path.basename(sys.path[0]) == "tests":
+    sys.path.pop(0)
+
+from decnet.prefork import run_fleet  # noqa: E402
+
+
+def main(out: str) -> None:
+    tick_log = os.path.join(out, "tick.log")
+    crash_log = os.path.join(out, "crash.log")
+
+    def tick() -> None:
+        while True:
+            with open(tick_log, "a") as f:
+                f.write("t\n")
+            time.sleep(0.2)
+
+    def crasher() -> None:
+        with open(crash_log, "a") as f:
+            f.write("c\n")
+        time.sleep(0.15)
+        os._exit(1)
+
+    # Fast backoff so we observe multiple restarts inside the short window.
+    run_fleet(
+        {"tick": tick, "crasher": crasher},
+        max_backoff=0.2,
+        poll_interval=0.05,
+        stop_after=2.0,
+    )
+
+
+if __name__ == "__main__":
+    main(sys.argv[1] if len(sys.argv) > 1 else ".")