Add rolling speed and turn-rate metrics to debug HUD

[Coldaine]

Adds two rolling metrics to the debug HUD overlay in ViewController.swift:

• linearMetersPerSecond — rolling 2-second average from cumulative distanceTravelled delta
• turnDegreesPerSecond — rolling 2-second average from accumulated yaw deltas (rad→deg), with shortestAngleDeltaRadians() handling ±π wrapping

Both computed in updateDebugMotionMetrics() and rendered in statsUpdated.

[Copilot]

Pull request overview

Adds rolling motion metrics to the iOS debug HUD so developers can see recent (windowed) speed and turn rate while mapping/visualizing.

Changes:

• Introduces a small rolling sample buffer (MotionSample) and a 2-second window constant for debug motion metrics.
• Computes rolling average linear speed and turn rate in updateDebugMotionMetrics().
• Renders the new metrics in the statsUpdated debug HUD text.

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[Copilot]

updateDebugMotionMetrics() is called on every statsUpdated callback even when debugShown is false, but the resulting metrics are only used inside the debug HUD block. Consider gating the sampling/metric computation behind debugShown (and optionally clearing debugMotionSamples when debug is hidden) to avoid extra per-frame work on the native callback thread.

Suggested change

	let motionMetrics = self.updateDebugMotionMetrics(timestamp: now, distanceTravelled: distanceTravelled, yaw: yaw)
	let motionMetrics = self.debugShown ? self.updateDebugMotionMetrics(timestamp: now, distanceTravelled: distanceTravelled, yaw: yaw) : nil

[Copilot]

The rolling window uses Date().timeIntervalSince1970 as the sample timestamp. Since wall-clock time can jump (NTP/manual change), deltaTime and the purge window can become incorrect (including negative deltas clamped to 0.001, or samples not being purged for a while). Use a monotonic clock (e.g., ProcessInfo.processInfo.systemUptime or CACurrentMediaTime()) for these motion metrics instead.

Suggested change

	let now = Date().timeIntervalSince1970
	let now = ProcessInfo.processInfo.systemUptime