[android/fs] external content loader + nca/xci patches (#3596)

Foreword: WHY DON'T EVERYBODY USE ONE FOLDER FOR EACH GAME+CONTENTS? AIN'T THIS THE FORMAT GAMES COME WHEN YOU BUE THEM? DO YOU LIVE WITH ALL YOUR FRIENDS AND HAVE A 2ND HOUSE FOR ALL THE CHILDREN?

Nice, i feel better now.

This feat extends Maufeat's work on external content loading. It harmonically additions:
"...also, if in each game folder X, you find a folder Y, and in this folder Y you detect ONLY a single game, then mount all external content for that game found in that folder Y and its subfolders."
Permanent (not toggleable). External Content folders are supported equally.

Also:
-Reworked several routines for preserving single source of truth between android and other systems;
-Fixed the annoying unknown format error for content files, by providing proper format detection.

Reviewed-on: https://git.eden-emu.dev/eden-emu/eden/pulls/3596
Reviewed-by: MaranBr <maranbr@eden-emu.dev>
Reviewed-by: Lizzie <lizzie@eden-emu.dev>
Co-authored-by: xbzk <xbzk@eden-emu.dev>
Co-committed-by: xbzk <xbzk@eden-emu.dev>

src/core/loader/loader.h

@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: Copyright 2026 Eden Emulator Project
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 // SPDX-FileCopyrightText: Copyright 2018 yuzu Emulator Project
 // SPDX-License-Identifier: GPL-2.0-or-later
 
@@ -46,12 +49,29 @@ enum class FileType {
 };
 
 /**
- * Identifies the type of a bootable file based on the magic value in its header.
+ * Identifies the type of a supported file/container based on its structure.
  * @param file open file
  * @return FileType of file
  */
 FileType IdentifyFile(FileSys::VirtualFile file);
 
+/**
+ * Returns whether the file type represents a container format that can bundle multiple titles
+ * (currently NSP/XCI).
+ */
+bool IsContainerType(FileType type);
+
+/**
+ * Returns whether a container file is bootable as a game (has Application/Program content).
+ *
+ * @param file open file
+ * @param type optional file type; if Unknown it is auto-detected.
+ * @param program_id optional program id hint for multi-program containers.
+ * @param program_index optional program index hint for multi-program containers.
+ */
+bool IsBootableGameContainer(FileSys::VirtualFile file, FileType type = FileType::Unknown,
+                             u64 program_id = 0, std::size_t program_index = 0);
+
 /**
  * Guess the type of a bootable file from its name
  * @param name String name of bootable file