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>

tests/fail/old_extension.fch

@@ -1,3 +0,0 @@
-// expect: but Finch files now end in .fch
-import oldmodule
-fn main() {}

tests/fail/oldmodule.fn

@@ -1,2 +0,0 @@
-fn hi() {
-}