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/precache.test.ts

@@ -280,6 +280,7 @@ describe("Precache eviction integration", () => {
             pools: new RequestPools(),
             source,
             cacheDirectory: cacheDir,
+            downloadDirectory: join(root, "photos"),
             getFile: (id) => byID.get(id),
             statfs,
             cacheOriginalsMaxBytes: 25, // holds two 10-byte originals
@@ -348,6 +349,7 @@ describe("Precache preemption", () => {
             pools: new RequestPools({ contentConcurrency: 1 }),
             source,
             cacheDirectory: join(root, "cache"),
+            downloadDirectory: join(root, "photos"),
             getFile: (id) => byID.get(id),
             statfs: async () => ({ bsize: 1, bavail: 1_000_000_000 }),
             freeBelowBytes: 0,