feat(mapper): add SendDirectUpdate method for direct MapResponse delivery

[roganlynch]

Adds SendDirectUpdate method to the mapper batcher interface for direct MapResponse delivery.

Changes

• Extends the Batcher interface with SendDirectUpdate(ctx context.Context, node *types.Node, update *tailcfg.MapResponse) error
• Implements the method to deliver MapResponse updates directly to specific nodes without queuing
• Provides foundation for sending targeted ping requests to nodes

Context

This is a prerequisite for the ping functionality, allowing the control plane to send MapResponses containing ping requests directly to target nodes. This builds upon the mapper infrastructure and enables immediate delivery of control messages.

Part of the work discussed in #2902 to add ping functionality to headscale.

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Note: This code was generated with assistance from Claude Sonnet 4.5 via cline.bot