fix: kernel concurrency, interrupt context, and user VA safety

programs/man/framebuffer.2

@@ -21,8 +21,9 @@
     than width * 4 due to alignment). bpp is always 32.
 
 .SS fb_map
-    Maps the physical framebuffer into the process address space at
-    a fixed virtual address (0x50000000) and returns that address.
+    Maps the physical framebuffer into a dedicated region of the process
+    address space and returns its address. Programs must use the returned
+    pointer rather than assuming a particular virtual address.
 
         uint32_t* pixels = (uint32_t*)montauk::fb_map();