feat(stdlib): add Os.* — the OS/environment interface (#21)

An Os.* namespace, Go-flavored and game-scoped, for the environment *around*
the game: the command line, environment variables, standard streams, process
exit, the host platform, and the per-user known folders a game writes into.
Rounds the bare System.* builtins (arg/getenv/exit) into one coherent surface.

  - args / arg_count / arg     the argument vector (args() -> []string)
  - env / env_or / has_env     read env vars (null-safe via env_or)
  - set_env / unset_env        mutate this process's environment
  - exit(code)                 terminate with a status code
  - platform() / arch()        host facts (uname sysname/machine)
  - stdout_write / stderr_write  raw writes to the standard streams
  - save_dir / config_dir / cache_dir / temp_dir   per-user known folders

Pure libc over NUL-terminated strings; C-free, no new runtime. arg_count/arg/
exit stay light (no prelude) as thin aliases of the existing intrinsics; the
rest share one Os runtime prelude emitted on demand (g_uses_osrt). platform()
is portable (uname system name is field 0 on every Unix); arch() and the
known-folder layout follow the macOS/BSD conventions — the fully supported
native target today. Linux/Windows/wasm folder resolution and a target-aware
arch() are documented follow-ups.

- examples/library/os.ludic: asserts the invariants that hold regardless of
  host — env round-trip, env_or fallback, unset, args()==arg_count(), non-empty
  platform/arch and known dirs. Wired into `x test` (now 54 passed).
- docs: a new Os section + 17 per-symbol pages; inventory updated; every fence
  passes check-docs (--fmt) and the site builds via docgen.
- seed regenerated; `x bootstrap-cfree` fixpoint holds.

Closes #21

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/language/os/os-set_env.md

@@ -0,0 +1,26 @@
+---
+id: os-set_env
+name: Os.set_env
+category: os
+kind: namespace-method
+tokens: Os.set_env
+sig: Os.set_env(name, value) -> bool
+tip: Set an environment variable.
+order: 7
+ns: Os
+member: set_env
+---
+
+Sets an environment variable for this process (and any children it launches), overwriting any existing value. Returns <code>true</code> on success. Environment changes are process-local and non-deterministic host state — keep them out of the simulation.
+
+Parameters:
+- `name` — the variable name
+- `value` — the value to set
+
+```ludic
+program SetEnv {
+  entry {
+    if Os.set_env("LUDIC_MODE", "fast") { print(1) }
+  }
+}
+```