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.)

src/host/config.ts

@@ -30,6 +30,7 @@ export interface HostConfig<TTag extends string, Instance, RootCtx> {
     ctx: RootCtx,
   ): void;
   emit?(type: string, id: string, payload: unknown): void;
+  finalizeInstance?(instance: Instance, ctx: RootCtx): void;
 }
 
 export interface Root<TTag extends string, Instance, RootCtx> {
@@ -70,6 +71,7 @@ export function createRoot<TTag extends string, Instance, RootCtx>(
         effect: null,
         signalDisposers: [],
         prevProps: null,
+        disposed: false,
       };
       if (parentFiber) parentFiber.children.push(fiber);
       return fiber;
@@ -106,6 +108,7 @@ export function createRoot<TTag extends string, Instance, RootCtx>(
       effect: null,
       signalDisposers: [],
       prevProps: null,
+      disposed: false,
     };
 
     for (const child of el.children) {
@@ -132,6 +135,7 @@ export function createRoot<TTag extends string, Instance, RootCtx>(
         effect: null,
         signalDisposers: [],
         prevProps: null,
+        disposed: false,
       };
     }
 
@@ -158,6 +162,7 @@ export function createRoot<TTag extends string, Instance, RootCtx>(
       effect: null,
       signalDisposers: [],
       prevProps: null,
+      disposed: false,
     };
 
     for (const child of el.children) {
@@ -281,6 +286,7 @@ export function createRoot<TTag extends string, Instance, RootCtx>(
         effect: null,
         signalDisposers: [],
         prevProps: null,
+        disposed: false,
       };
       const payloadChildren = isURoot(node) ? (node as URoot).children : [node];
       for (const child of payloadChildren) {