Workstream C: relocate video seek-strategy logic into player core

Move the pure determine_video_seek_strategy function and its VideoSeekStrategy
enum (plus the 5 seek-strategy unit tests) out of the command layer into
player/seek.rs, where they belong and are testable without the Tauri State
harness. commands/player.rs now imports them from crate::player. No behavior
change.

src-tauri/src/player/mod.rs

@@ -7,6 +7,7 @@ pub mod backend;
 pub mod events;
 pub mod media;
 pub mod queue;
+pub mod seek;
 pub mod session;
 pub mod sleep_timer;
 pub mod state;
@@ -27,6 +28,7 @@ pub use backend::{NullBackend, PlayerBackend, PlayerError};
 pub use events::{PlayerEventEmitter, PlayerStatusEvent, TauriEventEmitter};
 pub use media::{MediaItem, MediaSource, MediaType, QueueContext};
 pub use queue::{QueueManager, RepeatMode};
+pub use seek::{determine_video_seek_strategy, VideoSeekStrategy};
 pub use session::{MediaSessionManager, MediaSessionType};
 pub use sleep_timer::{SleepTimerMode, SleepTimerState};
 pub use state::{EndReason, PlayerState};