docs: first install shows the run as printed

The Batocera example is dated and carries the Not installable line and the closing list of files the collection does not hold, which every Batocera run prints. Disk sizes are no longer hand-kept: the run prints its size. The no-platform case describes the menu install.sh makes possible by handing over the terminal.

tests/test_doc_pages.py

@@ -668,5 +668,63 @@ class PublishedNumbersShareOneFormat(unittest.TestCase):
         self.assertEqual(re.findall(r"\*\*\d{4,}\b[^*]*\*\*", readme), [])
 
 
+class FirstInstallWalkthrough(unittest.TestCase):
+    """First install showed a Batocera run of 12893 files with no omission
+    line and said any other ending meant work left undone, while every
+    Batocera run printed 399 files not installed. Its disk sizes (Batocera
+    4.0 GB) contradicted its own example (8.6 GB), and its no-platform case
+    said a piped command cannot ask, where install.sh hands it the terminal
+    and the installer shows a menu."""
+
+    def setUp(self):
+        self.text = _page("first-install.md")
+
+    def test_no_hand_kept_size_before_the_run(self):
+        import install
+
+        section = _section(self.text, "Before starting")
+        # The confirmation threshold is a constant of the installer, not a pack size.
+        self.assertEqual(install.CONFIRM_FROM_BYTES, 1024 ** 3)
+        self.assertIn("reaches 1 GB", section)
+        self.assertNotRegex(section.replace("reaches 1 GB", ""), r"\d+(\.\d+)? ?[KMG]B\b")
+
+    def test_the_run_is_dated_and_consistent_with_the_manifest(self):
+        import install
+
+        self.assertRegex(self.text, r"published on \d{4}-\d{2}-\d{2}")
+        step3 = _section(self.text, "Step 3. Let it work")
+        step4 = _section(self.text, "Step 4. Read the end of the run")
+        step5 = _section(self.text, "Step 5. Confirm it later")
+        count, size = re.search(r"\n  (\d+) files \(([\d.]+ [KMG]B)\)", step3).groups()
+        self.assertIn(f"0/{count} present", step3)
+        self.assertIn(f"Downloading {count} files ({size})", step3)
+        self.assertIn(f"Done. {count} files installed", step4)
+        self.assertIn(f"{count}/{count} present ({count} verified", step5)
+        manifest = json.loads((ROOT / "install" / "batocera.json").read_text(encoding="utf-8"))
+        omitted = re.search(r"Not installable: (\d+) files \((\d+) required", step3)
+        if manifest.get("omitted_files"):
+            self.assertIsNotNone(omitted, "the manifest omits files and the run shows no line for it")
+        if omitted:
+            total, required = map(int, omitted.groups())
+            first, needed = install._omission_lines(
+                total, [{"required": True, "dest": str(i)} for i in range(required)]
+            )[:2]
+            self.assertIn(first, step4)
+            self.assertIn(needed.split(":")[0], step4)
+            self.assertIn(f"all {required} belong", step4)
+            self.assertIn(f"other {total - required} are optional", step4)
+
+    def test_no_platform_detected_reads_like_the_installer(self):
+        source = (ROOT / "install.py").read_text(encoding="utf-8")
+        branch = source.split('print("  No supported platform detected.")', 1)[1][:400]
+        self.assertIn("if not interactive or listing:", branch)
+        self.assertIn("_prompt_manual_platform(os_type)", branch)
+        self.assertIn('"$INSTALLER" "$@" </dev/tty', (ROOT / "install.sh").read_text(encoding="utf-8"))
+        section = _section(self.text, "No supported platform detected.")
+        self.assertIn("a menu follows", section)
+        self.assertIn("--no-input", section)
+        self.assertNotIn("cannot stop and ask", section)
+
+
 if __name__ == "__main__":
     unittest.main()