[MG_Pipe, MG_Remote, MG_Impl] (P7): the ID-49 neutral read_pixels pack has one spelling

- MG_Pipe/MGPipeTypes.h gains inline MGPipeNeutralReadPixelsPack() next to MGPPixelPackState, whose owner it is; the applier that installs the neutral state and the comparator whose oracle it is now call the same function
- MG_Remote/Server/PipeApplier.cpp read_pixels takes the constant from there instead of building it field by field
- MG_Impl/Pipe/PipeFill.cpp drops ServerReadPixelsNeutralPack, the re-typed second copy: a drift between two hand-typed spellings costs a FALSE DIVERGENCE on the server read, not a build break, which is the direction that fails quietly
- notes/p7/verify-split.md 2.2 says one spelling and why the old reasoning (do not reach into MG_Remote/Server from MG_Impl) did not survive the failure-direction argument

MobileGL/MG_Impl/Pipe/PipeFill.cpp

@@ -681,19 +681,12 @@ namespace MobileGL::MG_Pipe {
                    self.CurrentVerb() == MGPipeVerb::ReadPixels;
         }
 
-        // PipeApplier.cpp's `neutralPack`, field for field. A second spelling of a constant is
-        // the price of not reaching into MG_Remote/Server from here; if the two ever part, the
-        // server-side read stops matching this and the lane goes red by name, which is the
-        // direction that fails loudly.
-        PixelStoreParameters ServerReadPixelsNeutralPack() {
-            PixelStoreParameters neutral{};
-            neutral.RowLength = 0;
-            neutral.SkipRows = 0;
-            neutral.SkipPixels = 0;
-            neutral.SkipImages = 0;
-            neutral.Alignment = 1;
-            return neutral;
-        }
+        // The constant itself is MG_Pipe's (MGPipeTypes.h's MGPipeNeutralReadPixelsPack), which is
+        // the same function MG_Remote/Server/PipeApplier.cpp's read_pixels installs. It used to be
+        // re-typed here, field for field, to avoid reaching into MG_Remote/Server from MG_Impl -
+        // but the owner of MGPPixelPackState is MG_Pipe, both sites already include it, and a
+        // drift between the two spellings would have cost a false divergence rather than a build
+        // break.
 #endif // MOBILEGL_BUILD_DISAGGREGATED
 #endif // MOBILEGL_PIPE_VERIFY
     } // namespace
@@ -740,7 +733,7 @@ namespace MobileGL::MG_Pipe {
             g_verify.InHook = true;
             PipeInputs::VisitStorage(field, g_readScratch, g_readScratch, [](auto& live, auto&) {
                 if constexpr (std::is_same_v<std::remove_reference_t<decltype(live)>, PixelStoreParameters[2]>) {
-                    live[0] = ServerReadPixelsNeutralPack();
+                    live[0] = MGPipeNeutralReadPixelsPack().Pack;
                 }
                 return true;
             });

MobileGL/MG_Pipe/MGPipeTypes.h

@@ -1139,6 +1139,31 @@ namespace MobileGL::MG_Pipe {
     static_assert(sizeof(MGPPixelPackState) == 28,
                   "MGPPixelPackState changed size; update the wire format and this assertion");
 
+    // ID-49's NEUTRAL PACK, in one place. The server's read_pixels reads the backend with this
+    // state and scatters the tight reply through the application's own pack on the client side
+    // (MG_Remote/Server/PipeApplier.cpp), so the pack state never shapes the wire answer.
+    //
+    // ONE SPELLING, because there are two readers of the same constant and they are in different
+    // libraries: the applier that installs it, and (in a verify build) the compare-at-read hook in
+    // MG_Impl/Pipe/PipeFill.cpp, whose oracle for the pack half inside that window IS this value.
+    // Two hand-typed copies would drift silently in the ONE direction that matters - the hook
+    // would stop recognising the window and start reporting a divergence that is not one - so
+    // MG_Pipe, which owns MGPPixelPackState, owns the constant as well.
+    //
+    // Value-initialised and then the five fields the applier sets: RowLength / SkipRows /
+    // SkipPixels / SkipImages are already 0 by PixelStoreParameters' own defaults and are spelled
+    // out anyway, because "neutral" is a statement about every member rather than about the ones
+    // whose default happens to be right.
+    inline MGPPixelPackState MGPipeNeutralReadPixelsPack() {
+        MGPPixelPackState neutral{};
+        neutral.Pack.RowLength = 0;
+        neutral.Pack.SkipRows = 0;
+        neutral.Pack.SkipPixels = 0;
+        neutral.Pack.SkipImages = 0;
+        neutral.Pack.Alignment = 1;
+        return neutral;
+    }
+
     // Also a shader-variant input: both backends bake these into the synthesized
     // pass-through control stage.
     struct MGPPatchState {

MobileGL/MG_Remote/Server/PipeApplier.cpp

@@ -452,12 +452,10 @@ namespace MobileGL::MG_Remote::Server {
         // in which the pack state is neutral does not outlive the call.
         const MG_Pipe::PixelStoreParameters savedPack =
             MG_Pipe::gPipeInputs.GetPixelStoreParameters(/*isUnpack=*/false);
-        MG_Pipe::MGPPixelPackState neutralPack{};
-        neutralPack.Pack.RowLength = 0;
-        neutralPack.Pack.SkipRows = 0;
-        neutralPack.Pack.SkipPixels = 0;
-        neutralPack.Pack.SkipImages = 0;
-        neutralPack.Pack.Alignment = 1;
+        // MG_Pipe owns the constant (MGPipeTypes.h): in a verify build the compare-at-read hook's
+        // oracle for the pack half inside this window is the same value, and a second hand-typed
+        // copy would drift without a build break.
+        const MG_Pipe::MGPPixelPackState neutralPack = MG_Pipe::MGPipeNeutralReadPixelsPack();
         MG_Pipe::MGPipeApplySetPixelPackState(neutralPack);
 
         table->GL.ReadPixels(info.Box.X, info.Box.Y, static_cast<GLsizei>(info.Box.W),

docs/Disaggregated/notes/p7/verify-split.md

@@ -128,8 +128,10 @@ upload，所以描述符宣告 `(target, level)` 而调用递空指针，会走
 `#if MOBILEGL_BUILD_DISAGGREGATED`）。** 只在这个窗口里换预言：server stamp 之下、`ReadPixels` 动词上，pack 半边
 的期望值是 applier 读用的中性 pack；unpack 半边和其它每个字段仍以活上下文为预言。整字段比较失败后，把 scratch
 里的 pack 半边换成中性值**再比一次整字段**——pack 既不是应用的值、也不恰为中性值的 server 读，仍是分歧、仍 Fatal。
-中性常量是 `PipeApplier.cpp` 的 `neutralPack` 的第二种拼法（不从这里伸手进 `MG_Remote/Server`）；两者一旦分叉，
-server 侧读就不再等于它，车道按名变红——失败方向是响的。
+中性常量**只有一种拼法**：`MG_Pipe/MGPipeTypes.h` 的 `MGPipeNeutralReadPixelsPack()`，applier 的 `read_pixels`
+与这里的预言是同一个函数的两个调用点。它一开始是逐字段重打的第二份（为了不从 `MG_Impl` 伸手进 `MG_Remote/Server`），
+但 `MGPPixelPackState` 的属主本来就是 `MG_Pipe`、两边本来就 include 它，而两份拼法分叉的代价是**一次假分歧**而不是
+一次编译错误——失败方向是哑的，所以改成一处。
 
 **放弃了什么，写明。** 在 server 的 `ReadPixels` 上，比对器看不见一个推送了**错误** pack 状态的 client，因为
 applier 在 backend 读之前把它覆盖了；消费真实 pack 的 client 侧 scatter 由 readback 矩阵用例按字节覆盖，不按字段。