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/lis-move-detection.test.ts

@@ -14,6 +14,7 @@ function makeFiber(key: string | undefined, tag: string): Fiber<string> {
     effect: null,
     signalDisposers: [],
     prevProps: null,
+    disposed: false,
   };
 }