Add pipeline executor with progress reporting and cancellation

Sequential execution with per-image progress callbacks, cancellation
via atomic flag, batch result tracking (success/fail/sizes/timing).
Phase 5 complete - 85 tests passing, zero clippy warnings.

pixstrip-core/src/lib.rs

@@ -2,6 +2,7 @@ pub mod config;
 pub mod discovery;
 pub mod encoder;
 pub mod error;
+pub mod executor;
 pub mod loader;
 pub mod operations;
 pub mod pipeline;