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>

src/battery.cpp

@@ -0,0 +1,35 @@
+#include "battery.hpp"
+#include <algorithm>
+#include <fstream>
+#include <string>
+#include <system_error>
+#include <vector>
+
+namespace frameyap {
+namespace {
+std::string first_line(const std::filesystem::path& path) {
+    std::ifstream in(path);
+    std::string line;
+    if (in) std::getline(in, line);
+    return line;
+}
+} // namespace
+
+std::optional<BatteryLevel> system_battery(const std::filesystem::path& root) {
+    std::error_code error;
+    std::vector<std::filesystem::path> supplies;
+    for (const auto& entry : std::filesystem::directory_iterator(root, error)) supplies.push_back(entry.path());
+    std::sort(supplies.begin(), supplies.end()); // deterministic choice among several
+    for (const auto& supply : supplies) {
+        if (first_line(supply / "type") != "Battery" || first_line(supply / "scope") == "Device") continue;
+        const auto capacity = first_line(supply / "capacity");
+        if (capacity.empty() || capacity.size() > 3 ||
+            !std::all_of(capacity.begin(), capacity.end(), [](char c) { return c >= '0' && c <= '9'; }))
+            continue;
+        const int percent = std::stoi(capacity);
+        if (percent > 100) continue;
+        return BatteryLevel{percent, first_line(supply / "status") == "Charging"};
+    }
+    return std::nullopt;
+}
+} // namespace frameyap