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>

boot/main.fch

@@ -42,11 +42,6 @@ struct Loader {
 fn load(ptr[Loader] l, str path, str module) {
     if !file_exists(path) {
         if module.len > 0 {
-            old := path.sub(0, path.len - 4) + ".fn"
-            if file_exists(old) {
-                print("error: found " + old + ", but Finch files now end in .fch: rename it to " + path)
-                exit(1)
-            }
             print("error: can't find the module '" + module + "' (looked for " + path + ")")
         } else {
             print("error: can't open '" + path + "'")