fix(partitioning): export partition variables for module access

Export ESP_PART and ROOT_PART variables to ensure they are
available in all subsequent modules (encryption, filesystems,
postinstall). While sourcing makes them accessible, explicit
exports clarify the sharing model and prevent potential issues
in different shell contexts.

Decision: Use explicit export instead of relying on implicit
variable propagation through source to make dependencies clear.

src/partitioning.sh

@@ -83,6 +83,7 @@ partition_disk() {
 
   ESP_PART="$(partition_path "$DISK" 1)"
   ROOT_PART="$(partition_path "$DISK" 2)"
+  export ESP_PART ROOT_PART
 
   log_info "ESP partition: $ESP_PART"
   log_info "Root partition: $ROOT_PART"