OpenGL and Vulkan examples; ptr(int)/int(ptr); trailing commas in calls; drop .fn hint

- examples/opengl.fch: OpenGL 3.3 core, shaders, VAO/VBO, a spinning triangle (checked offscreen)
- examples/vulkan.fch: instance, GPUs with properties/memory/queues, logical device and queue
- ptr(n) and int(p) conversions for C APIs that pass offsets as pointers
- .ptr works on string literals; a comma after the last call argument is allowed
- a missing header included by an imported header is now named in the error
- the old .fn extension hint is gone (Finch is new, there are no old files)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

shell.nix

@@ -10,9 +10,11 @@ pkgs.mkShell {
     ninja
     pkg-config              # finds the right flags for `link "..."`
 
-    # for examples/window.fch and examples/raylib.fch
+    # for the graphics examples (window, opengl, raylib, vulkan)
     glfw
     libGL
     raylib
+    vulkan-headers
+    vulkan-loader
   ];
 }