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,
     };
   }
 

test/fiber.test.ts

@@ -1,5 +1,6 @@
 import { describe, it, expect, expectTypeOf } from "vitest";
-import type { Fiber, Effect } from "../src/host/fiber.js";
+import type { Fiber, Effect, HostLike } from "../src/host/fiber.js";
+import { disposeFiber } from "../src/host/fiber.js";
 
 describe("Fiber<I> interface", () => {
   it("has all required fields", () => {
@@ -13,6 +14,7 @@ describe("Fiber<I> interface", () => {
       effect: null,
       signalDisposers: [],
       prevProps: null,
+      disposed: false,
     };
     expect(fiber.instance).toBe("inst-1");
     expect(fiber.tag).toBe("div");
@@ -36,6 +38,7 @@ describe("Fiber<I> interface", () => {
       effect: null,
       signalDisposers: [],
       prevProps: null,
+      disposed: false,
     };
     expect(fiber.key).toBeUndefined();
   });
@@ -51,6 +54,7 @@ describe("Fiber<I> interface", () => {
       effect: null,
       signalDisposers: [],
       prevProps: null,
+      disposed: false,
     };
     const child: Fiber<string> = {
       instance: "child-inst",
@@ -62,6 +66,7 @@ describe("Fiber<I> interface", () => {
       effect: null,
       signalDisposers: [],
       prevProps: null,
+      disposed: false,
     };
     parent.children.push(child);
     expect(child.parent).toBe(parent);
@@ -80,6 +85,7 @@ describe("Fiber<I> interface", () => {
       effect: null,
       signalDisposers: [() => disposed.push("a"), () => disposed.push("b")],
       prevProps: null,
+      disposed: false,
     };
     fiber.signalDisposers.forEach((d) => d());
     expect(disposed).toEqual(["a", "b"]);
@@ -109,6 +115,7 @@ describe("Effect<I> union type", () => {
       effect: null,
       signalDisposers: [],
       prevProps: null,
+      disposed: false,
     };
     const effect: Effect<string> = { type: "insert", before };
     expect(effect.type).toBe("insert");
@@ -126,6 +133,7 @@ describe("Effect<I> union type", () => {
       effect: null,
       signalDisposers: [],
       prevProps: null,
+      disposed: false,
     };
     const effect: Effect<string> = { type: "move", before };
     expect(effect.type).toBe("move");
@@ -153,6 +161,7 @@ describe("Effect<I> union type", () => {
       effect: { type: "update", payload: "x" },
       signalDisposers: [],
       prevProps: {},
+      disposed: false,
     };
     expect(fiber.effect!.type).toBe("update");
 
@@ -184,9 +193,242 @@ describe("Fiber re-export from barrel", () => {
       effect: null,
       signalDisposers: [],
       prevProps: null,
+      disposed: false,
     };
     const _effect: _EffectCheck = { type: "remove" };
     expect(_fiber.instance).toBe("inst");
     expect(_effect.type).toBe("remove");
   });
+});
+
+describe("disposeFiber", () => {
+  it("disposes a 3-level tree bottom-up: children before parent", () => {
+    const finalized: string[] = [];
+    const disposed: string[] = [];
+    const ctx = {};
+
+    const host: HostLike<string, typeof ctx> = {
+      finalizeInstance(instance, _ctx) {
+        finalized.push(instance);
+      },
+    };
+
+    const grandchild: Fiber<string> = {
+      instance: "gc",
+      tag: "span",
+      props: {},
+      key: undefined,
+      children: [],
+      parent: null as unknown as Fiber<string>,
+      effect: null,
+      signalDisposers: [() => disposed.push("gc-signal")],
+      prevProps: null,
+      disposed: false,
+    };
+
+    const child: Fiber<string> = {
+      instance: "child",
+      tag: "div",
+      props: {},
+      key: undefined,
+      children: [grandchild],
+      parent: null as unknown as Fiber<string>,
+      effect: { type: "update", payload: null },
+      signalDisposers: [() => disposed.push("child-signal")],
+      prevProps: null,
+      disposed: false,
+    };
+
+    const root: Fiber<string> = {
+      instance: "root",
+      tag: "root",
+      props: {},
+      key: undefined,
+      children: [child],
+      parent: null,
+      effect: null,
+      signalDisposers: [() => disposed.push("root-signal")],
+      prevProps: null,
+      disposed: false,
+    };
+
+    child.parent = root;
+    grandchild.parent = child;
+
+    disposeFiber(root, host, ctx);
+
+    expect(finalized).toEqual(["gc", "child", "root"]);
+    expect(disposed).toEqual(["gc-signal", "child-signal", "root-signal"]);
+  });
+
+  it("idempotent: double-dispose does not error or double-call", () => {
+    const finalized: string[] = [];
+    const disposed: string[] = [];
+    const ctx = {};
+
+    const host: HostLike<string, typeof ctx> = {
+      finalizeInstance(instance) {
+        finalized.push(instance);
+      },
+    };
+
+    const fiber: Fiber<string> = {
+      instance: "inst",
+      tag: "div",
+      props: {},
+      key: undefined,
+      children: [],
+      parent: null,
+      effect: { type: "update", payload: null },
+      signalDisposers: [() => disposed.push("sig")],
+      prevProps: null,
+      disposed: false,
+    };
+
+    disposeFiber(fiber, host, ctx);
+    disposeFiber(fiber, host, ctx);
+
+    expect(finalized).toEqual(["inst"]);
+    expect(disposed).toEqual(["sig"]);
+    expect(fiber.signalDisposers).toEqual([]);
+    expect(fiber.parent).toBeNull();
+    expect(fiber.effect).toBeNull();
+  });
+
+  it("signal subscriptions are cleaned up (effect no longer fires after dispose)", () => {
+    let callCount = 0;
+    const ctx = {};
+
+    const host: HostLike<string, typeof ctx> = {};
+
+    const disposer = () => { callCount = -1; };
+
+    const fiber: Fiber<string> = {
+      instance: "inst",
+      tag: "div",
+      props: {},
+      key: undefined,
+      children: [],
+      parent: null,
+      effect: null,
+      signalDisposers: [disposer],
+      prevProps: null,
+      disposed: false,
+    };
+
+    disposeFiber(fiber, host, ctx);
+
+    expect(fiber.signalDisposers).toEqual([]);
+    expect(callCount).toBe(-1);
+  });
+
+  it("does not call removeChild on host", () => {
+    const ctx = {};
+    const operations: string[] = [];
+
+    const host: HostLike<string, typeof ctx> = {
+      finalizeInstance() {
+        operations.push("finalizeInstance");
+      },
+    };
+
+    const fiber: Fiber<string> = {
+      instance: "inst",
+      tag: "div",
+      props: {},
+      key: undefined,
+      children: [],
+      parent: null,
+      effect: null,
+      signalDisposers: [],
+      prevProps: null,
+      disposed: false,
+    };
+
+    disposeFiber(fiber, host, ctx);
+
+    expect(operations).toEqual(["finalizeInstance"]);
+  });
+
+  it("works when finalizeInstance is undefined (no host cleanup)", () => {
+    const disposed: string[] = [];
+    const ctx = {};
+
+    const host: HostLike<string, typeof ctx> = {};
+
+    const fiber: Fiber<string> = {
+      instance: "inst",
+      tag: "div",
+      props: {},
+      key: undefined,
+      children: [],
+      parent: null,
+      effect: null,
+      signalDisposers: [() => disposed.push("sig")],
+      prevProps: null,
+      disposed: false,
+    };
+
+    expect(() => disposeFiber(fiber, host, ctx)).not.toThrow();
+    expect(disposed).toEqual(["sig"]);
+  });
+
+  it("clears fiber.parent after disposal", () => {
+    const ctx = {};
+    const host: HostLike<string, typeof ctx> = {};
+
+    const parent: Fiber<string> = {
+      instance: "parent",
+      tag: "div",
+      props: {},
+      key: undefined,
+      children: [],
+      parent: null,
+      effect: null,
+      signalDisposers: [],
+      prevProps: null,
+      disposed: false,
+    };
+
+    const child: Fiber<string> = {
+      instance: "child",
+      tag: "span",
+      props: {},
+      key: undefined,
+      children: [],
+      parent,
+      effect: null,
+      signalDisposers: [],
+      prevProps: null,
+      disposed: false,
+    };
+
+    parent.children.push(child);
+
+    disposeFiber(child, host, ctx);
+
+    expect(child.parent).toBeNull();
+  });
+
+  it("clears fiber.effect after disposal", () => {
+    const ctx = {};
+    const host: HostLike<string, typeof ctx> = {};
+
+    const fiber: Fiber<string> = {
+      instance: "inst",
+      tag: "div",
+      props: {},
+      key: undefined,
+      children: [],
+      parent: null,
+      effect: { type: "update", payload: "x" },
+      signalDisposers: [],
+      prevProps: null,
+      disposed: false,
+    };
+
+    disposeFiber(fiber, host, ctx);
+
+    expect(fiber.effect).toBeNull();
+  });
 });

test/key-matching-algorithm.test.ts

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

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,
   };
 }