Position-based mouse handler for agent-specific UI regions.

Only intercepts mouse events that require agent-specific handling:

• Prompt area clicks (split pane and side panel): focus/unfocus the input field and transition to insert mode.
• Preview pane scroll (split pane): scroll the file viewer preview when the mouse is in the preview column.
• Side panel interactions: scroll and click handling for the bottom panel, which uses UIState.scroll (not the standard viewport).

Chat content scroll in the split pane updates semantic transcript scroll state. Chat content clicks focus the agent pane, unfocus the prompt input, and pass through for the rest of the input stack.

Events outside agent regions pass through to the next handler in the surface stack.