fix(compiler): woc build -o creates the output's parent directory — a fresh checkout has no target/

- `woc build <dir> -o <example>/target/<name>` wrote its temp file beside the
  output and failed with "No such file or directory" when target/ was absent;
  target/ is gitignored, so every fresh checkout hit it — the db-actor and
  db-bench gates went red on a clean master worktree while passing on dev,
  where the directories exist from history
- the driver now creates the output's parent (mkdir -p shape) before the
  temp write; project-mode builds and existing directories are unchanged
- single-binary-smoke.sh gains `build-into-missing-dir` (4 checks, was 3),
  red on the old driver, green on this one; woc-test clean

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
(cherry picked from commit e9213bb949e1c26cb3a29d6f9babf2322a82d839)

compiler/bin/main.ml

@@ -878,6 +878,16 @@ let build_mode ?(deps : (string * string) list = []) ~(runtime : string option)
         Printf.eprintf "woc: %s\n" msg;
         exit 2
     in
+    (* a fresh checkout has no target/ directories: create the output's
+       parent, so `-o <dir>/<name>` works the way every gate and README
+       invokes it instead of failing on the temp file below *)
+    let rec mkdir_p d =
+      if d <> "" && d <> "." && d <> "/" && not (Sys.file_exists d) then begin
+        mkdir_p (Filename.dirname d);
+        (try Sys.mkdir d 0o755 with Sys_error _ -> ())
+      end
+    in
+    mkdir_p (Filename.dirname out);
     let tmp = out ^ ".woc-build.tmp" in
     (* stale tmp from an interrupted earlier build must not survive: its
        permission bits would leak through, since Open_creat on an

scripts/single-binary-smoke.sh

@@ -62,6 +62,21 @@ else
   ok "single-binary/build"
 fi
 
+# ---- -o into a directory that does not exist yet ---------------------------
+# A fresh checkout has no target/ anywhere (gitignored), and every example
+# gate builds with `-o <example>/target/<name>`: `woc build` must create the
+# output's parent rather than fail on its own temp file there.
+FRESH="$WORK/fresh/target/app"
+if ! timeout "$TIMEOUT" "$WOC" build "$SRC" -o "$FRESH" --runtime "$WOVM" \
+    >"$WORK/fresh.out" 2>"$WORK/fresh.err"; then
+  rc=$?
+  bad "single-binary/build-into-missing-dir" "woc build exited $rc: $(head -1 "$WORK/fresh.err")"
+elif [[ ! -x "$FRESH" ]]; then
+  bad "single-binary/build-into-missing-dir" "output missing or not executable: $FRESH"
+else
+  ok "single-binary/build-into-missing-dir"
+fi
+
 # ---- relocate outside the repo, run with no args, diff --------------------
 if [[ -x "$APP" ]]; then
   cp "$APP" "$ELSEWHERE/app"