feat: install one emulator's files in one command

install.py --emulator NAME keeps what the named emulators read on the detected platform: their own files and the platform's files of the systems they run, as a pack built for them carries, without the systems no core is mapped to. Each emulator page now gives that command for Linux, macOS and Windows, beside the per-file links.

tests/test_install.py

@@ -2902,6 +2902,39 @@ class ARefusedStandaloneCopyIsNamed(unittest.TestCase):
 
 
 
+class OneEmulatorsFilesInOneCommand(unittest.TestCase):
+    """An emulator page offers one command for that emulator's files.
+    --core kept the platform's whole list, and narrowing to cores kept the
+    platform files of every system no core is mapped to."""
+
+    MANIFEST = {
+        "system_cores": {"ibm-pc": ["pcem"], "nintendo-nes": ["fceumm"]},
+        "core_names": {"pcem": ["pcem"], "fceumm": ["fceumm"]},
+        "files": [],
+    }
+
+    def test_only_its_files_and_its_systems(self):
+        import contextlib  # noqa: PLC0415
+
+        files = [
+            {"dest": "pcem/ibmpc.bin", "cores": ["pcem"], "systems": ["ibm-pc"]},
+            {"dest": "disksys.rom", "cores": None, "systems": ["nintendo-nes"]},
+            {"dest": "ep128emu/exos.rom", "cores": None, "systems": ["enterprise"]},
+            {"dest": "ibm.rom", "cores": None, "systems": ["ibm-pc"]},
+        ]
+        with contextlib.redirect_stdout(io.StringIO()):
+            kept, _omitted = install._narrow_to_emulators(
+                ["PCem"], self.MANIFEST, "retroarch", files, [])
+        self.assertEqual(sorted(e["dest"] for e in kept), ["ibm.rom", "pcem/ibmpc.bin"])
+
+    def test_an_unknown_name_is_refused(self):
+        import contextlib  # noqa: PLC0415
+
+        with contextlib.redirect_stderr(io.StringIO()), self.assertRaises(SystemExit):
+            install._narrow_to_emulators(["nope"], self.MANIFEST, "retroarch", [], [])
+
+
+
 class ThePagesQuoteTheBootstrapsOwnWords(unittest.TestCase):
     """The installer page and the FAQ quoted "install.py SHA-256 mismatch",
     a message the bootstraps no longer print."""

tests/test_site_exports.py

@@ -818,5 +818,22 @@ class ACardSaysWhyAFileIsLeftOut(unittest.TestCase):
         self.assertIn("installed with the standalone emulator", badges)
 
 
+class AnEmulatorPageOffersItsInstall(unittest.TestCase):
+    """An emulator page linked each file one by one and offered no way to
+    install them together."""
+
+    def test_the_command_names_the_emulator(self):
+        from generate_site import _install_block
+
+        text = "\n".join(_install_block("pcem", {"type": "libretro"}))
+        self.assertIn("install.sh | sh -s -- --emulator pcem", text)
+        self.assertIn("install.ps1))) --emulator pcem", text)
+        self.assertEqual(_install_block("x", {"type": "alias"}), [])
+
+    def test_the_installer_takes_the_option(self):
+        source = (ROOT / "install.py").read_text(encoding="utf-8")
+        self.assertIn('"--emulator"', source)
+
+
 if __name__ == "__main__":
     unittest.main()