feat: add OpenGL abstraction layer with RAII resources

- Replace window class with cbt::opengl::context
- Add buffer resource (VBO, EBO, UBO, SSBO) with move semantics
- Add texture resource with format/type enums and filtering
- Add descriptor_set for Vulkan-style resource binding
- All resources use RAII with proper cleanup

deps/Findsokol.cmake

@@ -0,0 +1,56 @@
+# ==============================================================================
+# Findsokol.cmake
+# ==============================================================================
+# This module fetches the sokol single-header C libraries.
+# https://github.com/floooh/sokol
+#
+# Targets provided:
+#   sokol::sokol - Interface target (headers at repo root)
+#
+# Variables set:
+#   sokol_FOUND       - TRUE if sokol is available
+#   sokol_LIBRARIES   - The library target (sokol::sokol)
+#   sokol_INCLUDE_DIR - Include directory (repo root)
+# ==============================================================================
+
+if (DEFINED _FINDSOKOL_INCLUDED)
+    return()
+endif()
+set(_FINDSOKOL_INCLUDED TRUE)
+
+if (DEFINED sokol_FIND_VERSION AND NOT sokol_FIND_VERSION STREQUAL "")
+    set(SOKOL_TAG "${sokol_FIND_VERSION}")
+else()
+    set(SOKOL_TAG "HEAD")
+endif()
+
+message(STATUS "Fetching sokol @ ${SOKOL_TAG}")
+
+include(FetchContent)
+
+find_program(GIT_EXECUTABLE git)
+if (NOT GIT_EXECUTABLE)
+    message(FATAL_ERROR "Fetch with zip not supported.")
+endif()
+
+FetchContent_Declare(
+    sokol
+    GIT_REPOSITORY https://github.com/floooh/sokol.git
+    GIT_TAG        ${SOKOL_TAG}
+    GIT_SHALLOW    TRUE
+    SOURCE_SUBDIR  _unused  # no CMakeLists.txt here; skips add_subdirectory
+)
+
+# SOURCE_SUBDIR points to a non-existent directory so FetchContent_MakeAvailable
+# skips add_subdirectory; sokol is single-header files, we only need the root dir.
+FetchContent_MakeAvailable(sokol)
+
+add_library(sokol INTERFACE)
+target_include_directories(sokol SYSTEM INTERFACE "${sokol_SOURCE_DIR}")
+add_library(sokol::sokol ALIAS sokol)
+
+set(sokol_FOUND       TRUE)
+set(sokol_LIBRARIES   sokol::sokol)
+set(sokol_INCLUDE_DIR "${sokol_SOURCE_DIR}")
+
+set(SOKOL_LICENSE_FILE "${sokol_SOURCE_DIR}/LICENSE" CACHE FILEPATH "Path to sokol license file")