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

TODO.md

@@ -25,6 +25,17 @@ declares one.
 
 # Completed Steps
 
+- 2026-10-01: Each original's save path is
+  `YYYY/YYYY-MM/YYYY-MM-DD/YYYY-MM-DD.<fileID>.<ext>` under the library's
+  download directory, which defaults to `photos` in the working directory (issue
+  143). The date is the photo's `takenAt` in the machine's time zone.
+  `photo.savePath` is always a string, with or without a content cache, and
+  `isLocal` is true only when the original is at its save path.
+  `photo.download()` puts the original there, copied from the cache when the
+  cache holds it and fetched otherwise. `lib.backup()` does the same for each
+  file, writes each file's JSON beside its original, and links `collections/` to
+  the save paths; the backup has no `originals/` folder.
+
 - 2026-10-01: A `Photo` has `savePath`, `isLocal`, `content()`, `exif()`,
   `modifiedAt`, `hash` and `year` (issue 141). `savePath` is where
   `lib.backup()` writes the original under the library's download directory; for