Acknowledge serialized MCP browser commands

[CameronFoxly]

Summary

Adds the Ascii-Motion browser companion for #153 and the shared targeted cell-batch support needed by #152.

• serializes authoritative browser commands FIFO and emits exactly one correlated success or failure result
• applies set_cells_batch to an exact active-layer content-frame entry, including inactive entries, while keeping canvasStore synchronized for the installed frame
• applies FPS changes with timelineStore.setFrameRate(fps, false) and reflows legacy frame-duration sequences
• exposes awaitable raw-data session import for v1/v2 migration, validation, store restoration, and final active-canvas installation
• preserves legacy command and JSON-RPC notification inputs

Wire contract

Server to browser:

interface MCPCommandRequest {
  type: 'command_request';
  requestId: string;
  command: MCPCommand;
}

Browser to server:

type MCPCommandResult =
  | {
      type: 'command_result';
      requestId: string;
      success: true;
      applied?: {
        currentFrameIndex?: number;
        cellsChanged?: number;
        frameRate?: number;
        durationMs?: number;
      };
    }
  | {
      type: 'command_result';
      requestId: string;
      success: false;
      error: string;
    };

Acknowledged requests execute and emit results strictly FIFO, including async handlers. Supported command payloads added here are:

{ type: 'set_cells_batch'; cells: Array<{ x: number; y: number; cell: Cell }>; frameIndex?: number }
{ type: 'set_frame_rate'; fps: number }
{ type: 'set_frame_duration'; index: number; duration: number } // duration is milliseconds
{ type: 'load_project'; sessionData: unknown }

Dependency

This is the browser half of the cross-repository #153 fix. It can merge independently, but acknowledged live mutations require the pending CameronFoxly/ascii-motion-mcp #153 companion PR to emit command_request and await command_result using the exact contract above. The targeted set_cells_batch.frameIndex behavior is also the shared browser primitive for the ascii-motion-mcp #152 image-import companion; no import-specific browser command is introduced.

The navigation-specific atomic completion required by #154 is intentionally excluded and will be added in a stacked browser companion using this dispatcher's async executeCommand extension point.

Refs #153
Refs #152

Validation

• npm test -- --run src/__tests__/mcpClient.test.ts src/__tests__/sessionMigration.test.ts src/__tests__/phase6Integration.test.ts (58 tests)
• npm run build
• targeted ESLint on changed files