feat(installer): add safe Mac role selection policy

[Betafer]

Summary

• add a standalone hardware policy for deciding which Mac should become the display
• recognize the reference 2017 21.5-inch 4K iMac and Mac mini M4 exactly
• handle iMac, MacBook, Mac mini, headless, physical-size, and pixel-area heuristics
• ignore TargetBridge virtual displays when evaluating attached hardware
• require an explicit user choice whenever the available evidence is ambiguous
• expose non-technical role labels for the setup UI

Why

An automatic installer should not ask an inexperienced user to understand “Sender” and “Receiver”, but it also must not silently assign surprising roles. The policy makes strong, explainable choices only when the hardware evidence is clear and otherwise returns no recommendation.

Validation

• 26 focused checks pass through a standalone Swift test executable
• covers the reference iMac/Mac mini pair, iMac + MacBook, two similar notebooks, headless Macs, virtual-only displays, display size/resolution, Rosetta, OS minima, and ambiguous desktops
• tests assert that visible role copy avoids Sender/Receiver terminology

This PR contains only the pure role-selection/probing core and its tests; it does not add or activate an installer UI.