Document the cascade guarantee and the cascade ceiling

A node runs when something it reads was published, not merely when something
upstream of it ran — worth saying now that it is true, since it is what a
reader relies on when a rate-limited port goes quiet. Plus `FLOW_MAX_CASCADES`
in the engine's configuration table.

docs/concepts/flows.md

@@ -27,6 +27,10 @@ being explicit about the consequences:
 
 - **Fan-in is free.** Two nodes providing `temperature` are two producers of
   one message. The consumer does not change.
+- **A node runs when something it reads was published.** Not merely when
+  something upstream of it ran: a node that produced nothing this time — held
+  back by a rate limit, say — leaves what reads it on the value it already
+  has, and so does everything behind that.
 - **A wire cannot be wrong.** There is no wire. There is a name that either
   matches or does not, and the canvas tells you at edit time which it is.
 - **Layout is not a document.** The canvas computes the arrangement, so a flow