Add read-only Linux system battery reader

Reads the device's own battery from /sys/class/power_supply, skipping
peripheral (scope=Device) supplies and implausible values. Offline tested
against a fake power_supply tree.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/battery_test.cpp

@@ -0,0 +1,37 @@
+#include "battery.hpp"
+#include <cassert>
+#include <fstream>
+#include <string>
+#include <unistd.h>
+
+using namespace frameyap;
+namespace fs = std::filesystem;
+namespace {
+void put(const fs::path& path, const std::string& value) {
+    fs::create_directories(path.parent_path());
+    std::ofstream(path) << value << '\n';
+}
+}
+int main() {
+    const auto root = fs::temp_directory_path() / ("frameyap-battery-" + std::to_string(::getpid()));
+    fs::remove_all(root);
+    assert(!system_battery(root)); // missing tree
+    fs::create_directories(root);
+    put(root / "AC/type", "Mains");
+    put(root / "AC/online", "1");
+    put(root / "hid-controller/type", "Battery"); // paired peripheral, not the headset
+    put(root / "hid-controller/scope", "Device");
+    put(root / "hid-controller/capacity", "12");
+    assert(!system_battery(root));
+    put(root / "BAT0/type", "Battery");
+    put(root / "BAT0/capacity", "250"); // implausible values are ignored
+    assert(!system_battery(root));
+    put(root / "BAT0/capacity", "64");
+    put(root / "BAT0/status", "Discharging");
+    assert((system_battery(root) == BatteryLevel{64, false}));
+    put(root / "BAT0/status", "Charging");
+    assert((system_battery(root) == BatteryLevel{64, true}));
+    put(root / "BAT0/capacity", "x1");
+    assert(!system_battery(root));
+    fs::remove_all(root);
+}