Make 5bytes the default feature for idx entry compatibility

seaweed-volume/Cargo.toml

@@ -13,7 +13,7 @@ path = "src/main.rs"
 
 [features]
 # Match the default Go build: 4-byte offsets unless explicitly built with 5-byte support.
-default = []
+default = ["5bytes"]
 # Enable 5-byte offset mode for 8TB max volume size (matches Go builds with -tags 5BytesOffset).
 # Without this feature, uses 4-byte offsets with 32GB max volume size.
 5bytes = []