Save originals at photos/YYYY/YYYY-MM/YYYY-MM-DD/YYYY-MM-DD.fileID.ext; add photo.download() (closes #143)

Each original's save path is now
YYYY/YYYY-MM/YYYY-MM-DD/YYYY-MM-DD.<fileID><ext> under the library's
download directory, dated by takenAt in the machine's time zone. The
directory defaults to photos/ in the working directory, resolved once at
open. savePath is always a string, with or without a content cache, and
isLocal is true only when the original is there.

photo.download() puts the original at its save path: copied from the
cache when the cache holds it, fetched straight there otherwise.
lib.backup() uses the same code for each file, writes each file's JSON
beside it, and links collections/ to the save paths. The backup has no
originals/ folder.

Model: opus-5-5

test/library/read.test.ts

@@ -36,6 +36,7 @@ import {
     makeAlbumsAPI,
     makePhotosAPI,
     makeTimelineAPI,
+    type SavePathLookup,
     type TimelineGroup,
 } from "../../src/library/read.js";
 import { Library } from "../../src/library/index.js";
@@ -101,12 +102,19 @@ const file = (
 };
 
 // Build the three API objects over one fixed projection, the way `Library`
-// wires them over its live store.
+// wires them over its live store. Save paths are covered in
+// content-library.test.ts; these tests never ask for one.
 const apis = (records: DerivedRecords) => {
     const derive = () => records;
+    const saves: SavePathLookup = {
+        savePath: () => {
+            throw new Error("no save paths in these tests");
+        },
+        isLocal: () => false,
+    };
     return {
-        albums: makeAlbumsAPI(derive),
-        photos: makePhotosAPI(derive),
+        albums: makeAlbumsAPI(derive, saves),
+        photos: makePhotosAPI(derive, saves),
         timeline: makeTimelineAPI(derive),
     };
 };