Implement disposeFiber for fiber tree disposal

Add disposeFiber() function that performs bottom-up teardown of fiber
subtrees: recursively disposes children before parents, calls
host.finalizeInstance for per-instance cleanup, invokes signal disposers,
and clears fiber state. Idempotent via disposed flag. Does NOT call
host.removeChild (that's the commit phase's job).

- Add disposeFiber + HostLike to src/host/fiber.ts
- Add finalizeInstance to HostConfig interface
- Add disposed boolean to Fiber interface
- Export disposeFiber and HostLike from barrel
- Add 7 tests for disposeFiber (3-level tree, idempotency, signal cleanup, etc.)

test/commit-mutations.test.ts

@@ -210,6 +210,7 @@ describe("commitMutations direct", () => {
       effect: null,
       signalDisposers: [],
       prevProps: null,
+      disposed: false,
     };
   }