Add storage module for presets, config, session, and history persistence

Preset save/load/list/delete/import/export, config JSON persistence,
session state save/restore, and processing history log with append/clear.
All stored as JSON under ~/.config/pixstrip/.

pixstrip-core/src/lib.rs

@@ -7,6 +7,7 @@ pub mod loader;
 pub mod operations;
 pub mod pipeline;
 pub mod preset;
+pub mod storage;
 pub mod types;
 
 pub fn version() -> &'static str {