Redesign approach for CPU usage tracking

The previous implementation maintained three exponential moving averages
(1/5/15-minute) of CPU busy-fraction, sampled every 5 seconds. This had
two problems:

1. Wrong semantics: the metric was labelled "load_avg" (a Unix concept
   measuring run-queue depth), but the tracker actually measures CPU
   busy-time as a fraction of FreeRTOS ticks — i.e. utilization, not
   load.  The name was misleading.

2. Poor responsiveness: the 1-minute EMA (DECAY = exp(-5/60) ≈ 0.920)
   has a ~60-second time constant.  On a live web panel, this made the
   metric appear frozen, especially during short bursts of activity
   (e.g. an HTTP request from the panel itself).

This commit replaces the EMA with a 64-sample simple moving average
(SMA) over a compact uint8_t circular buffer. The sample interval is
60 s / 64 = 937,500 µs, so the window covers exactly 60 seconds:

Algorithm:
- Every 937,500 µs (= 60 s / 64), the FreeRTOS tick delta (busy vs
  idle ticks on core 0) is computed and stored as a uint8_t
  (0–255 = 0–100% utilization).
- The circular buffer holds 64 samples, spanning exactly 60 seconds
  (64 × 937,500 µs = 60,000,000 µs). The timer interval is derived
  directly from the window size: 60 000 000 / SMA_WINDOW µs.
- The index wraps with a bitwise AND (& 63) instead of modulo, since
  64 is a power of 2.
- The running sum is a uint16_t (max 64 × 255 = 16 320, fits easily).
- The result _sma_avg is a volatile float written atomically by the
  esp_timer task (core 0) and read from the main loop (core 1).
  No spinlock is needed: on Xtensa LX7, a 32-bit aligned float store
  is a single instruction.

The 60-second window smooths out short bursts (e.g. WiFi/HTTP spikes)
while reacting to sustained load changes within ~10–15 seconds.

Naming:
- The public API is now getCore0Util() returning a float in [0.0, 1.0].
  The name explicitly identifies which core is measured (core 0, which
  runs WiFi, MQTT, HTTP, and LwIP — see task_pinning.c).
- JSON keys: "load_avg" (array) → "core0_util" (scalar float, percent)
- HistorySample field: load_avg1_pct → core0_util_pct
- StatsHistory series key: "cpu_load" → "core0_util"
- Web panel label: "Load Avg" → "Core0 Util"

The history snapshot (once per minute) reads the same _sma_avg value,
which at that point represents the rolling average of the last 60
seconds — exactly one history interval.

examples/simple_repeater/MyMesh.cpp

@@ -1532,10 +1532,10 @@ void MyMesh::removeNeighbor(const uint8_t *pubkey, int key_len) {
 void MyMesh::formatStatsReply(char *reply, size_t reply_size) {
   snprintf(reply,
            reply_size,
-           "{\"battery_mv\":%u,\"uptime_secs\":%u,\"load_avg\":[%.2f,%.2f,%.2f],\"errors\":%u,\"queue_len\":%u}",
+           "{\"battery_mv\":%u,\"uptime_secs\":%u,\"core0_util\":%.1f,\"errors\":%u,\"queue_len\":%u}",
            getBatteryMilliVolts(true),
            _ms->getMillis() / 1000,
-           _cpu_tracker.getLoadAvg1(), _cpu_tracker.getLoadAvg5(), _cpu_tracker.getLoadAvg15(),
+           _cpu_tracker.getCore0Util() * 100.0f,
            _err_flags,
            _mgr->getOutboundTotal());
 }
@@ -1941,7 +1941,7 @@ void MyMesh::updateStatsHistory(unsigned long now_ms) {
       sample.heap_min = ESP.getMinFreeHeap();
       sample.psram_free = ESP.getFreePsram();
       sample.psram_min = ESP.getMinFreePsram();
-      sample.load_avg1_pct = (uint8_t)(_cpu_tracker.getLoadAvg1() * 100.0f + 0.5f);
+      sample.core0_util_pct = (uint8_t)(_cpu_tracker.getCore0Util() * 100.0f + 0.5f);
 #endif
       if (board.isExternalPowered()) sample.flags |= HISTORY_FLAG_EXTERNAL_POWER;
       if (board.isCharging()) sample.flags |= HISTORY_FLAG_CHARGING;
@@ -2532,7 +2532,7 @@ bool MyMesh::formatWebStatsSummaryJson(char* reply, size_t reply_size) {
                      "\"archive\":{\"logical\":\"%s\",\"available\":%s,\"path\":\"%s\",\"type\":\"%s\","
                      "\"total_bytes\":%llu,\"used_bytes\":%llu},"
                      "\"core\":{\"battery_mv\":%u,\"battery_pct\":%d,\"battery_display_pct\":%d,\"battery_min_mv\":%u,\"battery_max_mv\":%u,"
-                     "\"uptime_secs\":%lu,\"load_avg\":[%.2f,%.2f,%.2f],\"errors\":%u,\"queue_len\":%u,"
+                     "\"uptime_secs\":%lu,\"core0_util\":%.1f,\"errors\":%u,\"queue_len\":%u,"
                      "\"external_power\":%s,\"charging\":%s,\"vbus\":%s},"
                      "\"radio\":{\"noise_floor\":%d,\"last_rssi\":%.2f,\"last_snr\":%.2f,\"tx_air_secs\":%lu,\"rx_air_secs\":%lu},"
                      "\"packets\":{\"recv\":%u,\"sent\":%u,\"flood_tx\":%u,\"direct_tx\":%u,\"flood_rx\":%u,\"direct_rx\":%u,"
@@ -2565,7 +2565,7 @@ bool MyMesh::formatWebStatsSummaryJson(char* reply, size_t reply_size) {
                      battery_min_mv,
                      battery_max_mv,
                      static_cast<unsigned long>(uptime_millis / 1000),
-                     _cpu_tracker.getLoadAvg1(), _cpu_tracker.getLoadAvg5(), _cpu_tracker.getLoadAvg15(),
+                     _cpu_tracker.getCore0Util() * 100.0f,
                      _err_flags,
                      static_cast<unsigned>(_mgr->getOutboundTotal()),
                      board.isExternalPowered() ? "true" : "false",