F3.1: std.cli os_args — real OS argv accessor via #foreign _NSGetArgv (examples/0716)

Add library/modules/std/cli.sx: a pure-sx command-line argument accessor
backed by the macOS C runtime (_NSGetArgv/_NSGetArgc), no compiler change.

  os_argc() -> s64
  os_args(buf: []string) -> []string

Zero heap, zero per-arg allocation: os_args fills a caller-provided buffer
(stack array) with string VIEWS over the process's own argv block, which
lives for the whole process. The returned slice header is a by-value stack
return; nothing touches context.allocator.

Documents the `sx run` reality: under `sx run <prog.sx> ...` the process
argv is the interpreter's argv (sx, run, prog.sx, ...), not a program's
logical args. This accessor reports the real process argv truthfully;
mapping to logical args is a later consumer concern (distribution P3.1).

Non-macOS platforms bail loudly (message + _exit) rather than returning a
silent empty.

examples/0716-modules-cli-argv.sx asserts only deterministic structural
invariants (argc >= 1, argv[0] non-empty, os_argc() == filled length).

examples/0716-modules-cli-argv.sx

@@ -0,0 +1,31 @@
+// Real OS-argv accessor from `modules/std/cli.sx` (#foreign _NSGetArgv).
+//
+// Only DETERMINISTIC structural invariants are asserted — the actual arg
+// contents depend on how the test is invoked (under `sx run` the process
+// argv is the interpreter's: ["sx", "run", "<this file>"]), so we never
+// pin exact strings:
+//   - argc >= 1            (every process has argv[0])
+//   - argv[0] is non-empty (the executable path)
+//   - os_argc() agrees with the filled slice length (no truncation)
+//
+// `buf` is a stack `[64]string`; `os_args` fills it with zero-copy views
+// over the C runtime's argv block — no heap, no per-arg allocation.
+
+#import "modules/std.sx";
+#import "modules/std/cli.sx";
+
+main :: () {
+    buf : [64]string = ---;
+    args := os_args(buf[0..64]);
+
+    if args.len >= 1 { print("argc>=1: ok\n"); }
+    else { print("argc>=1: FAIL ({})\n", args.len); }
+
+    if args.len >= 1 {
+        if args[0].len > 0 { print("arg0-nonempty: ok\n"); }
+        else { print("arg0-nonempty: FAIL\n"); }
+    }
+
+    if os_argc() == args.len { print("argc-consistent: ok\n"); }
+    else { print("argc-consistent: FAIL (os_argc={} len={})\n", os_argc(), args.len); }
+}

examples/expected/0716-modules-cli-argv.exit

@@ -0,0 +1 @@
+0

examples/expected/0716-modules-cli-argv.stderr

@@ -0,0 +1 @@
+

examples/expected/0716-modules-cli-argv.stdout

@@ -0,0 +1,3 @@
+argc>=1: ok
+arg0-nonempty: ok
+argc-consistent: ok