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

internal/chunker/chunker.go

@@ -33,9 +33,10 @@ type Chunker struct {
 	maxChunkSize int
 }
 
-// chunkSizeSpread is the FastCDC-recommended factor between the average
+// ChunkSizeSpread is the FastCDC-recommended factor between the average
 // chunk size and the minimum (avg/spread) and maximum (avg*spread) sizes.
-const chunkSizeSpread = 4
+// The largest chunk the chunker can emit is therefore avg*ChunkSizeSpread.
+const ChunkSizeSpread = 4
 
 // NewChunker creates a new chunker with the specified average chunk size.
 // The actual chunk sizes will vary between avgChunkSize/4 and avgChunkSize*4
@@ -45,8 +46,8 @@ func NewChunker(avgChunkSize int64) *Chunker {
 	// FastCDC recommends min = avg/4 and max = avg*4
 	return &Chunker{
 		avgChunkSize: int(avgChunkSize),
-		minChunkSize: int(avgChunkSize / chunkSizeSpread),
-		maxChunkSize: int(avgChunkSize * chunkSizeSpread),
+		minChunkSize: int(avgChunkSize / ChunkSizeSpread),
+		maxChunkSize: int(avgChunkSize * ChunkSizeSpread),
 	}
 }