Reject a blob_size_limit below the largest possible chunk (closes #167)

Validate only rejected blob_size_limit below chunk_size, but the chunker can emit chunks up to chunk_size times the FastCDC size spread (four times), and the packer puts a single chunk of any size into an otherwise empty blob. A limit between one and four times chunk_size therefore let a blob reach four times the configured maximum, with most blobs holding a single chunk and so exposing individual chunk lengths to anyone who can list the destination.

Validate now rejects blob_size_limit below chunk_size times the spread, reusing the chunker's one constant (now exported as ChunkSizeSpread) instead of a second literal. The rule is stated in the error text, the Validate comment, the README config table, config.example.yml, and the generated config template.

Model: opus-4-8

config.example.yml

@@ -304,6 +304,8 @@ storage_url: "rclone://las1stor1//srv/pool.2024.04/backups/heraklion"
 
 # Maximum blob size
 # Multiple chunks are packed into blobs up to this size
+# Must be at least four times chunk_size (the largest chunk the chunker can
+# emit); a smaller limit would let a single-chunk blob exceed it.
 # Supports: 1GB, 10G, 500MB, 1GiB, etc.
 # Default: 10GB
 #blob_size_limit: 10GB