refactor: one place decides which profiles answer

The verifier and the builder each resolved the profiles a run names, in
thirty-five lines that differed only in how they failed: one exits, the
other returns empty-handed. An alias is the same binary under another
name and a launcher only starts an emulator, so neither has requirements
of its own, and both refusals have to say the same thing.

common raises now and each caller chooses its own ending. Six tests hold
the refusals, one of them reading both sources so a copy cannot grow
back. The manifest's core-complement phase comes out of generate_manifest
in the same pass, 60 to 34.

Verified inert: manifests identical entry for entry, and the Handy pack
rebuilds to the same bytes.

scripts/common.py

@@ -838,6 +838,54 @@ def get_mame_clone_map() -> dict[str, str]:
 _emulator_profiles_cache: dict[tuple[str, bool], dict[str, dict]] = {}
 
 
+class ProfileSelectionError(ValueError):
+    """A named profile cannot answer for itself."""
+
+
+def select_emulator_profiles(
+    profile_names: list[str],
+    all_profiles: dict,
+    standalone: bool = False,
+) -> list[tuple[str, dict]]:
+    """Resolve named profiles, refusing the ones that answer for others.
+
+    An alias is the same binary under another name and a launcher only
+    starts an emulator, so neither has requirements of its own: naming one
+    is a question about the wrong profile, and the error says which one to
+    ask instead. Raising rather than exiting lets the verifier stop and the
+    builder return empty-handed from one rule.
+    """
+    selected: list[tuple[str, dict]] = []
+    for name in profile_names:
+        if name not in all_profiles:
+            available = sorted(
+                key
+                for key, value in all_profiles.items()
+                if value.get("type") not in ("alias", "test")
+            )
+            raise ProfileSelectionError(
+                f"emulator '{name}' not found\n"
+                f"Available: {', '.join(available[:10])}..."
+            )
+        profile = all_profiles[name]
+        kind = profile.get("type", "libretro")
+        if kind == "alias":
+            raise ProfileSelectionError(
+                f"{name} is an alias of {profile.get('alias_of', '?')} "
+                f"-use --emulator {profile.get('alias_of', '?')}"
+            )
+        if kind == "launcher":
+            raise ProfileSelectionError(
+                f"{name} is a launcher -use the emulator it launches"
+            )
+        if standalone and "standalone" not in kind:
+            raise ProfileSelectionError(
+                f"{name} ({kind}) does not support --standalone"
+            )
+        selected.append((name, profile))
+    return selected
+
+
 def load_emulator_profiles(
     emulators_dir: str,
     skip_aliases: bool = True,