feat: offer the one-liner where users land

The download page had no Android line and no Windows form with
options, and platform and system pages offered only ZIP parts. One
generator now writes the Linux, Windows and Android commands for the
download page and the platform, emulator and system pages. A system
section lists each platform's command under the system id its install
manifest records, the only names --system accepts.

tests/test_site_exports.py

@@ -882,6 +882,50 @@ class NestedProfileBlocksRenderAsLists(unittest.TestCase):
         self.assertIn("name: a.bin, size: 2", html)
 
 
+class TheOneLinerIsWhereUsersLand(unittest.TestCase):
+    """The download page had no Android line and no Windows form with
+    options, and the platform and system pages offered only ZIP parts."""
+
+    def test_download_page(self):
+        import generate_site  # noqa: PLC0415
+
+        page = generate_site.generate_which_pack()
+        self.assertIn("Android (Termux", page)
+        self.assertIn("install.ps1))) --check", page)
+        self.assertIn("--platform retroarch --dest", page)
+
+    def test_platform_and_system_pages(self):
+        import generate_site  # noqa: PLC0415
+        from cross_reference import FileConsumers  # noqa: PLC0415
+
+        cov = {"platform": "Batocera", "mode": "md5", "present": 0, "total": 0,
+               "verified": 0, "missing": 0, "details": [], "config": {"systems": {}}}
+        page = generate_site.generate_platform_page("batocera", cov)
+        self.assertIn("install.sh | sh -s -- --platform batocera", page)
+        self.assertIn("install.ps1))) --platform batocera", page)
+        self.assertLess(page.index("--platform batocera"), page.index("md-button"))
+
+        entry = {"name": "a.bin", "sha1": "a" * 40, "md5": "b" * 32, "size": 3,
+                 "path": "bios/Recalbox/MSX/a.bin"}
+        page = generate_site.generate_system_page(
+            "Microsoft", {"MSX": [entry]}, FileConsumers({"files": {}}),
+            {"MSX": [("recalbox", "msx1,msx2,msxturbor"), ("batocera", "msx1")]})
+        self.assertIn("--platform recalbox --system msx1,msx2,msxturbor\n", page)
+        self.assertIn("--platform batocera --system msx1", page)
+
+    def test_a_system_is_offered_only_where_the_manifest_records_it(self):
+        import generate_site  # noqa: PLC0415
+        import siterender  # noqa: PLC0415
+
+        saved = dict(siterender._system_page_map)
+        self.addCleanup(siterender.set_system_page_map, saved)
+        siterender.set_system_page_map({"sony-playstation": ("sony", "PlayStation"),
+                                        "nowhere": ("sony", "PlayStation")})
+        cov = {"retroarch": {"config": {"systems": {"sony-playstation": {}, "nowhere": {}}}}}
+        choices = generate_site.system_install_choices(cov)
+        self.assertEqual(choices["sony"]["PlayStation"], [("retroarch", "sony-playstation")])
+
+
 class TheGapPageOpensOnCounts(unittest.TestCase):
     """The page opened on a slogan and on tiles reading "Verified (100%)"
     for 5,437 of 5,438 files and "Missing 0", above a table of red cells."""