feat(lang): std.cli exit-code + --json contract helpers [F3.3]

Foundation milestone close — the minimal exit-code / --json contract
`dist` relies on, in pure sx (no compiler change).

- EX_OK (0) / EX_USAGE (64, sysexits.h) / EX_UNAVAILABLE (70) named
  constants in std.cli.
- exit_ok() / exit_usage() terminators routing through the canonical
  process.exit(code: u8) — removes the hand-rolled cli_bail_exit `_exit`
  binding; the unsupported-platform path now uses proc.exit(EX_UNAVAILABLE).
- --json read is parsed.json (already parsed by F3.2); documented as the
  detection point with a stdout-pure / stderr-human convention.
- examples/0718-modules-cli-exit-json.sx exercises the contract: json true
  with --json / false without, EX_USAGE == 64, and a usage path that exits
  64 via exit_usage() (expected .exit = 64).
- readme.md gains a std.cli command-line-interface subsection.

readme.md

@@ -425,6 +425,33 @@ The standard library (`modules/std.sx`) provides:
 - **Math**: `sqrt`, `sin`, `cos`
 - **Introspection**: `type_of`, `type_name`, `field_count`, `field_name`, `field_value`, `size_of`
 
+### Command-line interface (`modules/std/cli.sx`)
+
+`std.cli` builds command-line front-ends over an explicit logical argv
+(`[]string`): `os_args(buf)` reads the real process argv, and
+`parse(args, commands, diag) -> !Parsed` does subcommand dispatch + `--flag`
+parsing. On top of that it defines the small **exit-code / `--json` contract**
+a CLI program (e.g. `dist`) relies on:
+
+```sx
+#import "modules/std/cli.sx";
+
+p, e := parse(args, cmds, @diag);   // (Parsed, !CliError)
+if e == error.UnknownCommand {
+    log.err("unknown command '{}'", diag.token);   // human text -> stderr
+    exit_usage();                                   // usage error -> exit 64
+}
+if p.json { /* emit ONLY machine output on stdout */ }
+```
+
+- **Named exit codes** — `EX_OK` (0), `EX_USAGE` (64, the sysexits.h
+  command-line-usage code), `EX_UNAVAILABLE` (70, unsupported platform).
+- **Terminators** — `exit_ok()` / `exit_usage()` end the process with the
+  matching code; both route through the canonical `process.exit(code: u8)`.
+- **`--json` mode** — the reserved global `--json` flag surfaces as
+  `parsed.json` (true iff `--json` is in the argv). Convention: in json mode
+  stdout carries only the machine result; human diagnostics go to stderr.
+
 ## Cross-Compilation
 
 ```sh