fix: read the given database on every page

The page-finishing pass reloaded the repository's database.json whatever
--db said, so a card held only by the given collection raised KeyError.
siteinputs holds the inputs generate_site was given.

scripts/siteinputs.py

@@ -0,0 +1,32 @@
+"""The collection, profiles and platform files the site describes.
+
+generate_site.py takes them as --db, --emulators-dir and --platforms-dir and
+installs them here before writing a page. Every page module reads its inputs
+from current(), so a run on another collection describes that collection on
+every page. A module used on its own reads the repository's files.
+"""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+from pathlib import Path
+
+ROOT = Path(__file__).resolve().parent.parent
+
+
+@dataclass(frozen=True)
+class SiteInputs:
+    db: Path = ROOT / "database.json"
+    emulators: Path = ROOT / "emulators"
+    platforms: Path = ROOT / "platforms"
+
+
+_current = [SiteInputs()]
+
+
+def current() -> SiteInputs:
+    return _current[0]
+
+
+def use(inputs: SiteInputs) -> None:
+    _current[0] = inputs