Increment 3: battery health + power monitoring (IOKit)

- PowerInfo model + BatteryMath (health %, condition, watts, time fmt) — unit-tested
- PowerMonitor reads AppleSmartBattery via IORegistry (no root/entitlements):
  charge, cycles, design/raw-max capacity, temperature, voltage, amperage,
  time-to-empty/full, adapter watts; graceful no-battery (desktop) path
- Wire powerInfo into SystemMonitor (every 3rd cycle)
- Battery & Power section in Power & Storage tab; battery in menu-bar dropdown
- 10 new tests (42 total); cross-checked vs pmset/ioreg: health 96.9%, 114 cycles,
  30.8C, matches ground truth exactly

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

TopManagerTests/BatteryMathTests.swift

@@ -0,0 +1,58 @@
+import XCTest
+@testable import TopManager
+
+final class BatteryMathTests: XCTestCase {
+
+    func testHealthPercent() {
+        // 4000 mAh current full-charge vs 5000 mAh design = 80%
+        XCTAssertEqual(BatteryMath.healthPercent(maxCapacity: 4000, designCapacity: 5000), 80, accuracy: 0.001)
+    }
+
+    func testHealthPercentZeroDesignIsSafe() {
+        XCTAssertEqual(BatteryMath.healthPercent(maxCapacity: 100, designCapacity: 0), 0)
+    }
+
+    func testConditionNormalWhenHealthy() {
+        XCTAssertEqual(BatteryMath.condition(healthPercent: 92, cycleCount: 120), .normal)
+    }
+
+    func testConditionServiceWhenHealthLow() {
+        XCTAssertEqual(BatteryMath.condition(healthPercent: 74, cycleCount: 120), .serviceRecommended)
+    }
+
+    func testConditionServiceWhenCyclesHigh() {
+        XCTAssertEqual(BatteryMath.condition(healthPercent: 95, cycleCount: 1200), .serviceRecommended)
+    }
+
+    func testConditionUnknownWhenNoHealth() {
+        XCTAssertEqual(BatteryMath.condition(healthPercent: nil, cycleCount: 100), .unknown)
+    }
+
+    func testWattsPositiveWhenCharging() {
+        // 12 V * 2000 mA = 24 W
+        XCTAssertEqual(BatteryMath.watts(volts: 12, milliAmps: 2000), 24, accuracy: 0.001)
+    }
+
+    func testWattsNegativeWhenDischarging() {
+        XCTAssertEqual(BatteryMath.watts(volts: 12, milliAmps: -1500), -18, accuracy: 0.001)
+    }
+
+    func testFormatMinutes() {
+        XCTAssertEqual(BatteryMath.formatMinutes(135), "2h 15m")
+        XCTAssertEqual(BatteryMath.formatMinutes(45), "45m")
+        XCTAssertEqual(BatteryMath.formatMinutes(0), "—")
+    }
+
+    func testPowerInfoDerivedValues() {
+        let p = PowerInfo(
+            hasBattery: true, currentCharge: 66, isCharging: false, isPluggedIn: false,
+            fullyCharged: false, cycleCount: 150, designCapacity: 5000, maxCapacity: 4600,
+            temperature: 30.5, voltage: 12.0, amperage: -1500, timeToEmpty: 200,
+            timeToFull: nil, adapterWatts: nil
+        )
+        XCTAssertEqual(p.healthPercent ?? 0, 92, accuracy: 0.001)
+        XCTAssertEqual(p.condition, .normal)
+        XCTAssertEqual(p.powerWatts ?? 0, -18, accuracy: 0.001)
+        XCTAssertEqual(p.powerSourceLabel, "Battery Power")
+    }
+}