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.test.tsx

@@ -0,0 +1,34 @@
+/*
+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 { renderHook } from "@testing-library/react";
+import { describe, expect, test } from "vitest";
+
+import { useValueBehavior } from "./useValueBehavior";
+
+describe("useValueBehavior", () => {
+  test("holds the value from the first render", () => {
+    const { result } = renderHook(({ value }) => useValueBehavior(value), {
+      initialProps: { value: "a" },
+    });
+    expect(result.current.value).toBe("a");
+  });
+
+  test("keeps one identity and takes each new value", () => {
+    const { result, rerender } = renderHook(
+      ({ value }) => useValueBehavior(value),
+      { initialProps: { value: "a" } },
+    );
+    const value$ = result.current;
+    const seen: string[] = [];
+    value$.subscribe((value) => seen.push(value));
+
+    rerender({ value: "b" });
+    expect(result.current).toBe(value$);
+    expect(seen).toEqual(["a", "b"]);
+  });
+});