Add metadata stripping, watermark positioning, and rename template modules

Metadata: JPEG EXIF stripping (APP1 segment removal).
Watermark: 9-position grid calculation with margin support.
Rename: template parser with {name}, {ext}, {counter:N}, {width}, {height}
  and collision resolution with auto-suffix.
Phase 4 complete - 79 tests passing, zero clippy warnings.

pixstrip-core/src/operations/mod.rs

@@ -1,4 +1,7 @@
+pub mod metadata;
+pub mod rename;
 pub mod resize;
+pub mod watermark;
 
 use serde::{Deserialize, Serialize};