Keep a variant's accounting row when its file cannot be read (closes #340)

The reconciliation pass dropped a variant's accounting row on any error
reading its file's size, and logged every such file as missing. A file
still on disk then had no row, so eviction no longer counted it against
cache_max_bytes. Now only a missing file (ErrNotFound, which
FileStore.Size returns for it) loses its row, with the same info line.
Any other error keeps the row and logs a warning naming the variant key
and the error, and the pass goes on to the next key. README.md says so
under Storage.

Model: opus-5-5

TODO.md

@@ -30,6 +30,13 @@ P2: security: per-origin rate limiting
 
 # Completed Steps
 
+- 2026-10-26 the reconciliation pass drops a variant's accounting row only when
+  its file is missing (closes #340). Any other error reading the file's size
+  keeps the row. It dropped the row on any error and logged the file as missing.
+    - The kept row gets the warning "kept accounting row for unreadable variant
+      file", with the variant key and the error.
+    - The pass checks for `ErrNotFound`, which `FileStore.Size` returns for a
+      missing file.
 - 2026-10-26 on both image routes, a request is answered 499 and logged at info
   level when its client goes away before its image is ready, unless it is the
   request making the image (closes #339). It was answered 500 and logged at