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) {

src/host/fiber.ts

@@ -8,10 +8,34 @@ export interface Fiber<I> {
   effect: Effect<I> | null;
   signalDisposers: (() => void)[];
   prevProps: Record<string, unknown> | null;
+  disposed: boolean;
 }
 
 export type Effect<I> =
   | { type: "update"; payload: unknown }
   | { type: "insert"; before: Fiber<I> | null }
   | { type: "move"; before: Fiber<I> | null }
-  | { type: "remove" };
+  | { type: "remove" };
+
+export interface HostLike<I, Ctx> {
+  finalizeInstance?(instance: I, ctx: Ctx): void;
+}
+
+export function disposeFiber<I, Ctx>(fiber: Fiber<I>, host: HostLike<I, Ctx>, ctx: Ctx): void {
+  if (fiber.disposed) return;
+
+  for (const child of fiber.children) {
+    disposeFiber(child, host, ctx);
+  }
+
+  host.finalizeInstance?.(fiber.instance, ctx);
+
+  for (const disposer of fiber.signalDisposers) {
+    disposer();
+  }
+  fiber.signalDisposers = [];
+
+  fiber.parent = null;
+  fiber.effect = null;
+  fiber.disposed = true;
+}

src/mod.ts

@@ -19,7 +19,8 @@ export { ValuePointer, selectNode, setNode } from "./core/pointer.js";
 export { createRoot as createHostRoot } from "./host/config.js";
 export type { HostConfig, Root } from "./host/config.js";
 
-export type { Fiber, Effect } from "./host/fiber.js";
+export { disposeFiber } from "./host/fiber.js";
+export type { Fiber, Effect, HostLike } from "./host/fiber.js";
 
 export { scheduleUpdate, flushUpdates, reconcileProps, commitEffects, commitMutations, wireSignalToFiber, resetUpdateQueue, reconcileChildren, longestIncreasingSubsequence } from "./host/reconcile.js";
 export type { MatchedChild, ChildClassification, CommitContext } from "./host/reconcile.js";