chore: submodulos C++ en modo shallow (depth 1)

Marca todos los submodulos de cpp/vendor (y emsdk) como shallow en .gitmodules
para que los clones futuros descarguen solo el commit pinneado en vez de toda la
historia upstream. Reduce .git/modules de ~338MB a ~35MB sin perder codigo: el
working tree mantiene el snapshot completo de cada dependencia. Reversible por
submodulo con 'git fetch --unshallow'.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitmodules

@@ -1,22 +1,29 @@
 [submodule "cpp/vendor/imgui"]
 	path = cpp/vendor/imgui
 	url = https://github.com/ocornut/imgui.git
+	shallow = true
 	branch = docking
 [submodule "cpp/vendor/implot"]
 	path = cpp/vendor/implot
 	url = https://github.com/epezent/implot.git
+	shallow = true
 [submodule "cpp/vendor/tracy"]
 	path = cpp/vendor/tracy
 	url = https://github.com/wolfpld/tracy.git
+	shallow = true
 [submodule "cpp/vendor/glfw"]
 	path = cpp/vendor/glfw
 	url = https://github.com/glfw/glfw.git
+	shallow = true
 [submodule "cpp/vendor/implot3d"]
 	path = cpp/vendor/implot3d
 	url = https://github.com/brenocq/implot3d.git
+	shallow = true
 [submodule "cpp/vendor/sdl3"]
 	path = cpp/vendor/sdl3
 	url = https://github.com/libsdl-org/SDL.git
+	shallow = true
 [submodule "emsdk"]
 	path = emsdk
 	url = https://github.com/emscripten-core/emsdk.git
+	shallow = true