Add Director WebSocket sync docs and enable drag-and-drop survivor slot swapping in local BP UI

[luolangaga]

Motivation

• Provide a documented remote-control API so third-party apps can control ASG.Director over WebSocket without an OBS plugin.
• Improve local BP UX by allowing quick reordering of survivor slots via drag-and-drop.

Description

• Add DIRECTOR_SYNC_API.md which documents the ws://...:19625 WebSocket envelope format (protocol: asg-director-sync, version: 1, kind values), supported payload.data.type values, examples in JavaScript/Python, and operational notes.
• Add drag-and-drop swap support for survivor slots by adding styles (.slot-swap-tip, .slot.drag-over) to pages/css/local-bp.css.
• Implement initSurvivorSlotSwapDnD() and swapSurvivorSlotCharacter(fromIndex, toIndex) in pages/js/local-bp-logic.js, including drag event handlers, visual drag-over state, IPC calls via window.electronAPI.invoke('localBp:setSurvivor', ...), and updating state and UI; call initSurvivorSlotSwapDnD() from DOMContentLoaded.
• Update pages/local-bp.html to add a small tip (拖拽板块可以交换选角) above the selected-display grid.

Testing

• No automated tests were added or run for these changes.

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Codex Task