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/networking/fn-net_poll.md

@@ -0,0 +1,12 @@
+---
+id: fn-net_poll
+name: net_poll
+category: networking
+kind: builtin
+tokens: net_poll
+sig: net_poll(buf, cap) -> int
+tip: Receive queued bytes from the transport; returns the byte count.
+order: 1
+---
+
+Copy up to <code>cap</code> waiting bytes into <code>buf</code> and return how many arrived (0 when nothing is queued). Pair it with the generated <code>apply</code> to fold an update back into the world.