Merge a game with its updates and DLC while packing, without copying

[raiddevel]

🤔 Why?

• A game usually arrives in pieces: the base game, an update, sometimes a backport, and DLC, each one a folder or a RAR/ZIP.
• Packing them meant copying every piece over the base folder first (a second full copy of the game on disk, a long wait, and the original folders modified), then running mkpfs.

🔧 What changed

• mkpfs pack folder takes --layer PATH (repeatable, bottom to top). Each layer is a folder or a .rar/.zip; a file in a later layer replaces the same file in the layers below it, folders are merged, and the result is streamed straight into the usual exFAT-wrapped .ffpfsc builder.
• Nothing is copied and no source folder is modified. The image is byte-for-byte the one you would get from the hand-merged folder.
• The game folder inside each layer is detected (eboot.bin/sce_sys, a *-app folder, or the single folder inside); PATH::FOLDER picks it explicitly. Release notes (.nfo, .txt) next to it are not packed.
• Emulation builds still get their ampr_emu.index, generated in memory from the merged listing; a stale index found in any layer is replaced.
• --password applies to every encrypted archive among the layers. --raw is refused with layers (always exFAT-wrapped).

🧪 How to test

• mkpfs pack folder ./Game ./Game.ffpfsc --layer ./Game-Update --layer ./Game-DLC.zip
• mkpfs unpack --deep ./Game.ffpfsc ./out and compare with the folder you get by copying the update and the DLC over the game.
• uv run --frozen pytest tests/mkpfs/test_layered_source.py (folder+folder, folder+zip, zip base + folder layer, file/folder replacement, AMPR index, CLI round trip, and "the exFAT image of the layers equals the one of the hand-merged folder").
• Builds on the archive-source pull request (same ArchiveSubtree streaming, now behind a small SourceTree interface shared by archives and layers).

💬 Notes for non-technical readers

• Packing a single folder or a single archive is unchanged. Layers only come into play when --layer is given.
• The order matters: the last --layer is on top. Put the update after the game, and the DLC after the update, unless the DLC documentation says otherwise.
• A DLC that ships its own eboot.bin replaces the one below it, exactly as it would if you copied the folders by hand; mkpfs warns when the merged view ends up without eboot.bin + sce_sys/param.json at its root.
• The GUI does not expose layers yet; this pull request is CLI and library only.
• Used in the field by ps5-game-packer (Windows/Linux front end, GPL-3.0), which proposes the layer order from the file names and contents and lets the user reorder them before calling mkpfs.

Builds on #139 (its commits show here until that one is merged). Related to #138.