fix(artwork): let the overview row borrow the detail page's cover

A job added from the share extension opens on `amdl://download/<id>`, which
pushes the detail page straight onto the navigation stack — the overview
list never appears, so nothing ever requested its 256px cover. Backing out
of the detail page therefore landed on a row holding the type placeholder
until that fetch finished, even though the full-size cover was already
decoded and in the cache.

The two pages cache under different keys because they request different
sizes, and `fallbackCacheKey` only ever resolved one way: detail borrowing
the overview's small image. It is now symmetric — whichever size is missing
borrows the other. The row shows the hero image on its first frame (the
memory-cache lookup in `CachedAsyncImage.init` is synchronous), then swaps
in the 256 without a fade, because the fade keys on nil-to-non-nil and the
slot was never empty.

Nothing new is downloaded: this reuses an image already in the cache rather
than prefetching a second one ahead of time.

`fallbackCacheKey` moved onto `JobArtworkLoader` so the direction it picks
is testable, and the literal 256 became `overviewPixelSize` — it is part of
a cache key, not just a request size.

Private playlists are unaffected: both sizes share one unsized key, the two
lookups return the same string, and the fallback stays nil rather than
having a view borrow from itself.

Signed-off-by: LYJW131 <lyjw2007@gmail.com>
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Signed-off-by: LYJW131 <lyjw2007@gmail.com>

Author: LYJW131

amdl-ios/DownloadArtworkView.swift

@@ -8,6 +8,10 @@ import UIKit
 
 @MainActor
 enum JobArtworkLoader {
+    /// 概览列表那档尺寸。行里的槽位只有 56pt，但两个页面各存各的一份，所以它
+    /// 同时是一个缓存 key 的组成部分 —— 改动它会让所有已缓存的列表封面失效。
+    static let overviewPixelSize = 256
+
     /// 与详情页 Hero 封面一致：按显示原生像素的 2 倍请求并缓存大图。
     static var heroPixelSize: Int {
         let windowScenes = UIApplication.shared.connectedScenes.compactMap { $0 as? UIWindowScene }
@@ -29,6 +33,24 @@ enum JobArtworkLoader {
         return ""
     }
 
+    /// 本尺寸还没到位时可以先顶上的另一档尺寸的 key，没有可借的就是 nil。
+    ///
+    /// **两个方向都要**：
+    ///
+    /// - 概览 → 详情：大图还在下，先用列表里那张小的，免得 zoom 转场结束后短暂
+    ///   或永久露出任务类型占位图。
+    /// - 详情 → 概览：分享拓展和完成通知都是 `amdl://download/<id>` 深链，直接把
+    ///   详情页压进导航栈，概览列表一次都没出现过 —— 那张 256 于是从来没人取过。
+    ///   退回列表时若不借详情页已经下好的大图，就得从占位图重新等一次。
+    ///
+    /// 私人歌单两边共用一个不带尺寸的 key，两次算出来是同一个，这里返回 nil，
+    /// 不会自己借自己。
+    static func fallbackCacheKey(for job: Job, pixelSize: Int) -> String? {
+        let otherPixelSize = pixelSize == overviewPixelSize ? heroPixelSize : overviewPixelSize
+        let otherKey = cacheKey(for: job, pixelSize: otherPixelSize)
+        return otherKey == cacheKey(for: job, pixelSize: pixelSize) ? nil : otherKey
+    }
+
     static func prefetch(job: Job, pixelSize: Int) async {
         let primaryURL = job.artworkURL(pixelSize: pixelSize)
         if let request = PrivatePlaylistArtworkStore.request(for: job, pixelSize: pixelSize) {
@@ -49,7 +71,7 @@ enum JobArtworkLoader {
 /// 其他任务直接使用后端的 artwork_url。
 struct JobArtworkView: View {
     let job: Job
-    var pixelSize: Int = 256
+    var pixelSize: Int = JobArtworkLoader.overviewPixelSize
 
     @State private var fallbackURL: URL?
     @State private var artworkRevision = 0
@@ -70,11 +92,8 @@ struct JobArtworkView: View {
         JobArtworkLoader.cacheKey(for: job, pixelSize: pixelSize)
     }
 
-    /// 详情大图尚未准备好时先复用概览封面的缓存，避免 zoom 动画结束后
-    /// 短暂或永久露出任务类型占位图。
     private var fallbackCacheKey: String? {
-        let overviewKey = JobArtworkLoader.cacheKey(for: job, pixelSize: 256)
-        return overviewKey == cacheKey ? nil : overviewKey
+        JobArtworkLoader.fallbackCacheKey(for: job, pixelSize: pixelSize)
     }
 
     var body: some View {

amdl-iosTests/amdl_iosTests.swift

@@ -75,6 +75,53 @@ struct amdl_iosTests {
         try assert(small.imageCacheKey != large.imageCacheKey, "template image cache should retain size")
     }
 
+    /// 概览和详情各存各的尺寸，所以谁先拿到图，另一边都得能先借来顶上。
+    ///
+    /// 借的方向以前只有一个：详情借概览。分享拓展和完成通知走 `amdl://download/<id>`
+    /// 深链直接进详情页，概览列表压根没出现过，那张 256 从来没人取过 —— 退回列表
+    /// 时没得借，就得从占位图重新等一次。
+    @Test @MainActor func artworkFallsBackBetweenOverviewAndHeroSizes() throws {
+        let job = try DownloadsAPI.decodeDownloadDetail(from: """
+        {
+          "job": {
+            "id": "job_art", "input": "https://music.apple.com/cn/album/example/1", "type": "album",
+            "force": false, "status": "running", "total_items": 1, "done_items": 0, "failed_items": 0,
+            "artwork_url": "https://is1-ssl.mzstatic.com/image/thumb/x/{w}x{h}bb.jpg",
+            "created_at": "2026-07-30T00:00:00Z", "updated_at": "2026-07-30T00:00:00Z"
+          },
+          "items": []
+        }
+        """.data(using: .utf8)!).job
+
+        let overview = JobArtworkLoader.overviewPixelSize
+        let hero = JobArtworkLoader.heroPixelSize
+        try assert(hero != overview, "hero and overview must be different sizes for this to matter")
+
+        let overviewKey = JobArtworkLoader.cacheKey(for: job, pixelSize: overview)
+        let heroKey = JobArtworkLoader.cacheKey(for: job, pixelSize: hero)
+        try assert(overviewKey != heroKey, "each size caches separately")
+
+        // 详情 → 概览：这一条以前是 nil，正是深链进来后退回列表要等图的原因。
+        try assert(
+            JobArtworkLoader.fallbackCacheKey(for: job, pixelSize: overview) == heroKey,
+            "overview borrows the hero image"
+        )
+        // 概览 → 详情：原有方向，不能改坏。
+        try assert(
+            JobArtworkLoader.fallbackCacheKey(for: job, pixelSize: hero) == overviewKey,
+            "hero borrows the overview image"
+        )
+
+        // 私人歌单两档尺寸共用一个 key，没有另一份可借，不能自己借自己。
+        let privateJob = privatePlaylistJob(
+            artworkURL: "https://example-bucket.s3.amazonaws.com/cover.jpg?X-Amz-Expires=86400"
+        )
+        try assert(
+            JobArtworkLoader.fallbackCacheKey(for: privateJob, pixelSize: overview) == nil,
+            "a shared cache key has nothing to borrow"
+        )
+    }
+
     @Test func downloadDetailDecodesJobItems() throws {
         let json = """
         {