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>

tools/docgen/inventory.json

@@ -438,5 +438,24 @@
     "log-error",
     "log-set_level",
     "log-level"
+  ],
+  "os": [
+    "os-args",
+    "os-arg_count",
+    "os-arg",
+    "os-env",
+    "os-env_or",
+    "os-has_env",
+    "os-set_env",
+    "os-unset_env",
+    "os-exit",
+    "os-platform",
+    "os-arch",
+    "os-stdout_write",
+    "os-stderr_write",
+    "os-save_dir",
+    "os-config_dir",
+    "os-cache_dir",
+    "os-temp_dir"
   ]
 }

tools/x/selfhost.ludic

@@ -32,6 +32,7 @@ function selfhost_frags() -> []pointer {
   push(f, "selfhost/emit_uuid.ludic")
   push(f, "selfhost/emit_noise.ludic")
   push(f, "selfhost/emit_log.ludic")
+  push(f, "selfhost/emit_os.ludic")
   push(f, "selfhost/emit_list.ludic")
   push(f, "selfhost/emit_ease.ludic")
   push(f, "selfhost/emit_collide.ludic")

tools/x/test.ludic

@@ -103,6 +103,7 @@ function cmd_test() -> int {
   feat_case("library/uuid", "", "1 2 3 4 5 6 7 8 9 10", "uuid.ludic (Uuid v4/v7 format, version/variant, parse/equals)")
   feat_case("library/noise", "", "1 2 3 4 5 6 7 8 9 10 11", "noise.ludic (Noise value/perlin/simplex/fbm/cellular determinism + range)")
   feat_case("library/logging", "", "0 5 2 1", "logging.ludic (Log levels, set_level/level threshold, structured fields)")
+  feat_case("library/os", "", "1 2 3 4 5 6 7 8 9 10 11 12 13", "os.ludic (Os args/env round-trip, platform/arch, known dirs)")
 
   # issue #9: the Time/Date/Duration/Clock stdlib, driven from its own `entry`.
   net_case("lang/offline_rewards", "13 650 2026-08-30 0")