Remove mutable field from action types

Similar in spirit to #58938.

The app router reducer state used to be managed by useReducer, so it was
written to be resilient to rebasing — the same action may be processed
multiple times. Now that we've lifted the reducer outside of React
(#56497), each action runs only a single time. So we can simplify some
of the logic.

The purpose of the `mutable` field was so that if an action is
processed multiple times, state could be reused between each run; for
example, to prevent redundant network fetches. Now that this scenario
can no longer happen, we can remove it.

I had to update some of the unit tests in navigate-reducer because they
were written with the assumption that the reducer was called
multiple times. As far as I can tell, most of this behavior is covered
by e2e tests anyway, so I don't think it's too risky.

packages/next/src/client/components/app-router.tsx

@@ -162,7 +162,6 @@ function useServerActionDispatcher(dispatch: React.Dispatch<ReducerActions>) {
         dispatch({
           ...actionPayload,
           type: ACTION_SERVER_ACTION,
-          mutable: {},
         })
       })
     },
@@ -189,7 +188,6 @@ function useChangeByServerResponse(
           flightData,
           previousTree,
           overrideCanonicalUrl,
-          mutable: {},
         })
       })
     },
@@ -209,7 +207,6 @@ function useNavigate(dispatch: React.Dispatch<ReducerActions>): RouterNavigate {
         locationSearch: location.search,
         shouldScroll: shouldScroll ?? true,
         navigateType,
-        mutable: {},
       })
     },
     [dispatch]
@@ -329,7 +326,6 @@ function Router({
         startTransition(() => {
           dispatch({
             type: ACTION_REFRESH,
-            mutable: {},
             origin: window.location.origin,
           })
         })
@@ -344,7 +340,6 @@ function Router({
           startTransition(() => {
             dispatch({
               type: ACTION_FAST_REFRESH,
-              mutable: {},
               origin: window.location.origin,
             })
           })

packages/next/src/client/components/router-reducer/reducers/fast-refresh-reducer.ts

@@ -6,6 +6,7 @@ import type {
   ReadonlyReducerState,
   ReducerState,
   FastRefreshAction,
+  Mutable,
 } from '../router-reducer-types'
 import { handleExternalUrl } from './navigate-reducer'
 import { handleMutable } from '../handle-mutable'
@@ -18,16 +19,10 @@ function fastRefreshReducerImpl(
   state: ReadonlyReducerState,
   action: FastRefreshAction
 ): ReducerState {
-  const { mutable, origin } = action
+  const { origin } = action
+  const mutable: Mutable = {}
   const href = state.canonicalUrl
 
-  const isForCurrentTree =
-    JSON.stringify(mutable.previousTree) === JSON.stringify(state.tree)
-
-  if (isForCurrentTree) {
-    return handleMutable(state, mutable)
-  }
-
   mutable.preserveCustomHistoryState = false
 
   const cache: CacheNode = createEmptyCacheNode()