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-read_char.md

@@ -0,0 +1,12 @@
+---
+id: fn-read_char
+name: read_char
+category: builtins
+kind: builtin
+tokens: read_char
+sig: read_char() -> int
+tip: Read one byte from standard input (-1 at end of input).
+order: 24
+---
+
+Read a single byte from standard input, returning its value or <code>-1</code> at end of input — the basis for headless, pipe-driven runs.