docs: add Networking section + process/OS builtins (net_send, owner, is_server, serialize/apply, arg, exit, run, getenv, read_char, file_*)

Sourced from the compiler's confirmed builtin set and NETWORKING-DESIGN.md, so
the reference reflects the current language surface. 93 -> 104 symbols.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/language/builtins/fn-exit.md

@@ -0,0 +1,12 @@
+---
+id: fn-exit
+name: exit
+category: builtins
+kind: builtin
+tokens: exit
+sig: exit(code)
+tip: Exit the process immediately with a status code.
+order: 21
+---
+
+Terminate the process now with the given status code. Unlike <code>quit()</code>, which ends the game loop cleanly after the current frame, <code>exit</code> stops the program at once.