[release/5.0-preview4] Revert processing bundles in framework (#35679)v5.0.0-preview.4.20251.6

This commit reverts:
Revert "Single-File: Process bundles in the framework (#34274)"
This reverts commit 78b303df8fbb242985d049a277d0d199cafd51b5.

Revert "Single-File Bundler: Add a FileSize test (#35149)"
This reverts commit 779588a509b81d909ac5d496c172949ec1f1ddcc.

*Customer Scenario*

Publishing apps as a self-contained single-file doesn't work as expected.

* Publish needs to generate hostpolicy and hostfxr separate from the single file bundle
* Cross-platform publishing is incorrect

*Problem*

Since Static-apphost is not yet ready, processing bundle content in hostpolicy means that  hostpolicy and hostfxr DLLs need to be separate from the bundle. This causes self-contained single-file apps to not be a "single file" temporarily. 

The change also requires supporting changes from the SDK, to publish hostfxr and hostpolicy as separate files, and to invoke HostModel library with arguments that facilitate cross-platform publishing.

*Solution*

To solve these, problem, this change reverts:

Revert "Single-File: Process bundles in the framework (#34274)" commit 78b303df8fbb242985d049a277d0d199cafd51b5.

and a dependent test-only change:

Revert "Single-File Bundler: Add a FileSize test (#35149)" commit 779588a509b81d909ac5d496c172949ec1f1ddcc.

*Risk*

Medium
The change is contained to only host components: apphost, hostpolicy, and hostfxr.
However, the change is big, and needs testing in runtime and SDK repos.

*Testing*

Manually tested the SDK by inserting apphost, hostfxr, hostpolicy, and hostmodel library from this build into the `dotnet/packs` preview-4 SDK from https://github.com/dotnet/sdk/pull/11518 build.

Verified that:
* Singlefile apps can be published and run OK for  { Windows, Linux, Osx } x {netcoreapp3.0, netcoreapp3.1, netcoreapp5.0} 
* Cross-targeting builds of single-file apps build and run OK (ex: built on Windos, run on Mac).

1 files changed, 32 insertions, 0 deletions

diff --git a/src/installer/corehost/cli/apphost/bundle/marker.h b/src/installer/corehost/cli/apphost/bundle/marker.h
new file mode 100644
index 00000000000..52e185fbe79
--- /dev/null
+++ b/src/installer/corehost/cli/apphost/bundle/marker.h
@@ -0,0 +1,32 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+#ifndef __MARKER_H__
+#define __MARKER_H__
+
+#include <cstdint>
+
+namespace bundle
+{
+#pragma pack(push, 1)
+    union marker_t
+    {
+    public:
+        uint8_t placeholder[40];
+        struct
+        {
+            int64_t bundle_header_offset;
+            uint8_t signature[32];
+        } locator;
+
+        static int64_t header_offset();
+        static bool is_bundle()
+        {
+            return header_offset() != 0;
+        }
+    };
+#pragma pack(pop)
+
+}
+#endif // __MARKER_H__