Drop observable-hooks in favor of useBehavior

With this change, views now read every reactive value through `useBehavior`, as
`architecture.md` already prescribes. Where a view reads a plain stream, the view
model now exposes a `Behavior`.

src/useValueBehavior.ts

@@ -0,0 +1,25 @@
+/*
+Copyright 2026 Element Creations Ltd.
+
+SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE in the repository root for full details.
+*/
+
+import { useEffect } from "react";
+import { BehaviorSubject } from "rxjs";
+
+import { type Behavior } from "./state/Behavior";
+import { useInitial } from "./useInitial";
+
+/**
+ * React hook which mirrors a value from the render cycle into a behavior, so
+ * that a prop or memoised value can be handed to a view model. The behavior
+ * takes each new value once the render that produced it has committed.
+ */
+export function useValueBehavior<T>(value: T): Behavior<T> {
+  const value$ = useInitial(() => new BehaviorSubject(value));
+  useEffect(() => {
+    value$.next(value);
+  }, [value$, value]);
+  return value$;
+}