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/_section.md

@@ -1,7 +1,7 @@
 ---
 id: builtins
 title: Builtin functions
-order: 12
+order: 13
 ---
 
 Global functions available anywhere, beyond the namespaced APIs above.

docs/language/builtins/fn-arg.md

@@ -0,0 +1,12 @@
+---
+id: fn-arg
+name: arg_count / arg
+category: builtins
+kind: builtin
+tokens: arg_count arg
+sig: arg_count() -> int   arg(i) -> str
+tip: The command line; argv[0] is included.
+order: 20
+---
+
+Read the process command line: <code>arg_count()</code> is the number of arguments (including the program name at index 0) and <code>arg(i)</code> returns the <code>i</code>th as a string.

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.

docs/language/builtins/fn-file.md

@@ -0,0 +1,12 @@
+---
+id: fn-file
+name: file_write / file_stdout / file_stderr
+category: builtins
+kind: builtin
+tokens: file_write file_stdout file_stderr
+sig: file_stdout() -> ptr   file_stderr() -> ptr   file_write(handle, buf, len)
+tip: Low-level output handles and raw writing.
+order: 25
+---
+
+<code>file_stdout()</code> and <code>file_stderr()</code> return the standard stream handles; <code>file_write(handle, buf, len)</code> writes <code>len</code> raw bytes to one. Most code uses <code>print</code> instead — these are the primitive underneath.

docs/language/builtins/fn-getenv.md

@@ -0,0 +1,12 @@
+---
+id: fn-getenv
+name: getenv
+category: builtins
+kind: builtin
+tokens: getenv
+sig: getenv(name) -> str
+tip: Read an environment variable.
+order: 23
+---
+
+Return the value of environment variable <code>name</code> (empty when it is unset).

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.

docs/language/builtins/fn-run.md

@@ -0,0 +1,12 @@
+---
+id: fn-run
+name: run
+category: builtins
+kind: builtin
+tokens: run
+sig: run(cmd)
+tip: Run a shell command.
+order: 22
+---
+
+Run <code>cmd</code> as a shell command — handy for build steps and tooling written in Ludic.