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/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"
         )