Refuse a zero concurrency limit instead of reading it as the default

FLOW_MAX_WORKERS, FLOW_MAX_CASCADES and FLOW_MAX_RUNS are all pool sizes,
so 0 says neither "none" nor "unlimited" — it is a pool that cannot be
built. They are PositiveInt now, so a 0 fails at startup naming the
setting rather than being swallowed by `max_cascades or MAX_CASCADES`.
The consuming fallbacks take only None as "nobody said": explicit
`is None` in the executor, and no clamp on RunService.parallel.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01K1moruzue2kTJd3uVisgNk

backend/fluksio/flow/executor.py

@@ -46,6 +46,11 @@ DELAYED_INTERVAL_S = 1.0
 #: whose nodes wait on a network rather than a CPU may want more of them —
 #: `FLOW_MAX_CASCADES` is where that is said.
 MAX_CASCADES = 4
+#: Node threads, unless the service is given a number. Both this and the one
+#: above are taken as written: only ``None`` means "nobody said", so a number
+#: that reached here is one somebody chose, and an unusable one is the pool's
+#: ``ValueError`` rather than a silent 4.
+MAX_WORKERS = 4
 # How long a reload waits for claimed work to finish before rebuilding anyway.
 DRAIN_TIMEOUT_S = 10.0
 # Work waiting in the stream, undelivered. A burst is normal — the pool claims
@@ -68,7 +73,7 @@ class ExecutionService:
     ) -> None:
         self.queue = queue
         self._events = events
-        self.max_cascades = max_cascades or MAX_CASCADES
+        self.max_cascades = MAX_CASCADES if max_cascades is None else max_cascades
         self._pipeline: Pipeline | None = None
         self._stop = threading.Event()
         # Set when a deadline moves closer, so the timer thread stops waiting
@@ -82,7 +87,8 @@ class ExecutionService:
         # Entry ids claimed and still running, under _inflight_lock.
         self._active: set[str] = set()
         self.node_pool = ThreadPoolExecutor(
-            max_workers=max_workers or 4, thread_name_prefix="node"
+            max_workers=MAX_WORKERS if max_workers is None else max_workers,
+            thread_name_prefix="node",
         )
         self._cascade_pool = ThreadPoolExecutor(
             max_workers=self.max_cascades, thread_name_prefix="cascade"

backend/fluksio/flow/runs.py

@@ -769,7 +769,9 @@ class RunService:
         # the isolation it wants, minus surviving the process.
         self._state_factory = state_factory or (lambda _ns: MemoryState())
         self.engine_name = f"{socket.gethostname()}-{os.getpid()}"[:64]
-        self.parallel = max(1, parallel)
+        # Taken as written: clamping a 0 up to 1 would hide a limit somebody
+        # set, and the pool below rejects an unusable one loudly anyway.
+        self.parallel = parallel
         self._pool = ThreadPoolExecutor(
             max_workers=self.parallel, thread_name_prefix="run"
         )