cook: fail a build that stages no files

A recipe whose script exits 0 but installs nothing currently reports success,
and the damage lands far from the cause. qtspeech produced a 136-byte package
because upstream `return()`s with only a NOTICE when Qt6::Multimedia is absent;
that surfaced much later as an unresolved symbol in a consumer, not as a
qtspeech failure.

Zero regular files staged is unambiguous -- every real package installs at
least one file -- so this needs no per-recipe size threshold and cannot
false-positive on a small-but-valid package. The error names the likely cause
(a configure step soft-bailing on a missing optional dependency) rather than
just reporting emptiness. Metapackages that genuinely stage nothing opt out by
mentioning COOKBOOK_ALLOW_EMPTY_PACKAGE in their script.

This is the same failure shape as the libudev/libpciaccess API-surface risk: a
gap that reports success locally and only becomes visible as a link error in
something downstream.

src/cook/cook_build.rs

@@ -1179,6 +1179,49 @@ pub fn build(
                 globs.push((glob.compile_matcher(), stage_dir.clone()));
             }
         }
+        // EMPTY-PACKAGE GATE. A recipe whose script exits 0 but installs nothing
+        // is a build failure that currently reports success, and the damage
+        // lands far away: qtspeech produced a 136-byte package because upstream
+        // `return()`s with only a NOTICE when Qt6::Multimedia is missing, and
+        // that only surfaced much later as an unresolved symbol in a consumer.
+        //
+        // Zero regular files is unambiguous -- every real package installs at
+        // least one file -- so this needs no per-recipe threshold and cannot
+        // false-positive on a small-but-valid package. Recipes that legitimately
+        // stage nothing (metapackages) set COOKBOOK_ALLOW_EMPTY_PACKAGE=1.
+        {
+            fn count_files(dir: &Path) -> usize {
+                let mut n = 0;
+                if let Ok(entries) = fs::read_dir(dir) {
+                    for e in entries.flatten() {
+                        match e.file_type() {
+                            Ok(t) if t.is_dir() => n += count_files(&e.path()),
+                            Ok(_) => n += 1,
+                            Err(_) => {}
+                        }
+                    }
+                }
+                n
+            }
+            let staged_files = count_files(&stage_dir_tmp);
+            let allow_empty = matches!(
+                &recipe.build.kind,
+                BuildKind::Custom { script } if script.contains("COOKBOOK_ALLOW_EMPTY_PACKAGE")
+            );
+            if staged_files == 0 && !allow_empty {
+                return Err(format!(
+                    "{name}: build reported success but staged NO files.\n    \
+                     A package that installs nothing is a silent failure -- the error \
+                     surfaces later as a missing header or unresolved symbol in a \
+                     consumer, far from the cause.\n    \
+                     Check the recipe's configure step for a soft bail-out (a missing \
+                     optional dependency that turns the build into a no-op).\n    \
+                     If this package genuinely stages nothing, mention \
+                     COOKBOOK_ALLOW_EMPTY_PACKAGE in its script."
+                ));
+            }
+        }
+
         move_dir_all_fn(
             &stage_dir_tmp,
             &|path: PathBuf| {