The payoff of Phase FS-2: `fn main() -> Unit effect Log { Log::info("Hello,
world") }` compiles, runs freestanding (no libc, no ck_rt), prints `Hello, world`
via a real write syscall, exits 0.

- stdlib (promote into live tests/corpus/stdlib): StdoutLog.ck rewritten from six
  `= intrinsic` ops to real CkLang — `fn info(msg) { Sys::write(1, msg) }` (raw
  message; the level prefix needs String::concat, rides the operators arc). Log.ck
  + Log.ckh (effect Log + LogLevel/LogLine) promoted. String.ck/Bytes.ck stay in
  stdlib_old (String is builtin; both carry ck_rt-backed = intrinsic bodies that
  don't lower freestanding — the doc's Promotes list is aspirational for FS3).
- codegen/call.rs (new): the SHARED intrinsic-call site emit_intrinsic_call +
  emit_syscall_leaf — checks syscall_spec before intrinsic_target, emits
  `call ck_syscall(nr, …)`. Lifted out of the freestanding-only module so a
  syscall leaf emits uniformly from ck_entry, a user fn, or (later) an actor.
- codegen/freestanding.rs: emit_fn_main_object_freestanding now REUSES
  user_fn::emit_user_fns + emit_effect — only ck_entry + the LinkMode predicate
  (fn_main_is_freestanding: freestanding iff no reachable leaf is a ck_rt symbol)
  are freestanding-specific. THIN FORK held (201 lines).
- driver/compile/fn_main.rs (new): try_emit_fn_main + resolve_fn_main +
  lower_dispatched_providers (cross-module lowering of CkLang provider fns) +
  emit_by_link_mode. Split from compile/mod.rs (no LoC cap raised).
- Gate: tests/corpus/freestanding_log + freestanding_log_run.rs; in RUNTIME_TESTED
  + LIVE_STDLIB_APPS. Corpus reconcile self-closes the live stdlib.

Two-link-mode contract: Log/StdoutLog deliberately live in BOTH stdlib dirs during
the transition (= intrinsic form for the libc apps which have no ck_syscall; CkLang
form for freestanding) — FS8 deletes the = intrinsic form. stdlib_old/README.md
corrected to state this real invariant (was: "never hold the same live unit").

Observed: freestanding `Hello, world`, exit 0, 9040-byte static ELF, empty NEEDED,
empty `nm -u`, no ck_rt/printf/malloc/__libc. Libc path (helloWorld 2.1 MB,
greeter) confirmed green.

Gates (all green): fmt pass; clippy 0 warnings; cargo test --workspace exit 0 (no
libtest failures); R1-R4 arch caps pass (split, not raised); Docker e2e green.

Docs: stdlib.md §8 FS3 -> DONE; current-state.md FS3 note; stdlib_old/README.md.

@@ -111,3 +159,3 @@
 /// [`ContainerError`] if `docker` cannot be spawned. A non-zero *program* exit
 /// is **not** an error — it is the returned code (the program's value).
-pub fn exec(name: &str, bin: &str) -> Result<i32, ContainerError> {
+pub fn exec(name: &str, work_subdir: &str, bin: &str) -> Result<i32, ContainerError> {