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/run/c_pointers.fch

@@ -0,0 +1,22 @@
+import "string.h"
+
+struct Point {
+    int x
+    int y
+}
+
+fn main() {
+    p := Point(
+        x: 1,
+        y: 2,
+    )
+    print(p)
+    c := "finch".ptr
+    print(c[1], strlen("finch"))
+    q := ptr(4096)
+    print(int(q), int(ptr(0)) == 0, q != null)
+    a := [10, 20, 30]
+    address := int(a.ptr)
+    ptr[int] second = ptr(address + 8)
+    print(second.value)
+}

tests/run/c_pointers.out

@@ -0,0 +1,4 @@
+Point(x: 1, y: 2)
+i 5
+4096 true true
+20