#1638 chore: simplify mouse UI drag orchestration

Summary

• Complete the final R2-33 extraction so mouse UI simulation remains easier to review and maintain.
• Preserve all existing one-shot and incremental drag behavior while reducing the use case to a thin action dispatcher.

User Impact

• There is no intended user-visible behavior change.
• Drag target selection, response priority, animation timing, cancellation cleanup, and persistent drag state remain unchanged.

Changes

• Add a shared drag target resolver for path/raycast resolution and IDragHandler lookup.
• Reuse the resolver from one-shot drag and DragStart while preserving the observable failure order: start-target failure, drop-target failure, then no-handler fallback.
• Move DragStart, DragMove, DragEnd, and active-drag validation into MouseUiIncrementalDragExecutor.
• Keep MouseDragState ownership unchanged and pass the main-thread cleanup scheduler explicitly.
• Reduce SimulateMouseUiUseCase from 434 lines to 137 lines.

Verification

• Confirmed the resolver tests fail before implementation (0/3) and pass after implementation (3/3).
• Passed the focused EditMode suites (141/141).
• Passed the Simulate Mouse UI PlayMode suite before and after each extraction commit (32/32).
• Compiled the Unity package with 0 errors and 0 warnings.
• Compared the moved incremental drag block after normalizing static modifiers, scheduler arguments, and signatures; the only remaining difference was a trailing blank line.
• Verified both cleanup lambdas still call FinalizeDrag before MouseDragState.Clear with the same captures and timing.

Compatibility

• No IPC request or response shape changed, so the protocol version remains unchanged.
• R2-40 and R2-41 remain separate follow-up tasks.