ESP32: add CPUUsageTracker – core-0 load averages (1/5/15 min)

Track CPU utilisation on core 0 using a FreeRTOS tick hook that
increments per-tick idle/busy counters, sampled every 5s by an
esp_timer callback.  Exponential moving averages with Linux-style
time constants (1 / 5 / 15 min) are maintained in software:

  DECAY1  = exp(-5/60)  ≈ 0.9200  (1-minute window)
  DECAY5  = exp(-5/300) ≈ 0.9835  (5-minute window)
  DECAY15 = exp(-5/900) ≈ 0.9945  (15-minute window)

Core 1 is excluded intentionally: it runs the Arduino loopTask at
100% load for LoRa packet processing, so its figure is always 1.0
and carries no diagnostic value.  All other tasks are pinned to
core 0 by task_pinning.c, so core-0 load reflects the true system
utilisation.

New files:
  arch/esp32/CPUUsageTracker.h   – class declaration
  arch/esp32/CPUUsageTracker.cpp – tick hook + esp_timer sampling

Integration:
  MyMesh::begin() calls _cpu_tracker.begin() on ESP32 builds.
  formatStatsReply() emits "load_avg":[<1m>,<5m>,<15m>] in the
  compact stats JSON payload.
  formatWebStatsSummaryJson() adds the same field to the web-panel
  stats endpoint under core.load_avg.
  The web-panel HUD gains a "Load Avg" metric tile showing all
  three values side-by-side; the core-metrics grid is widened from
  4 to 5 columns.  The tile is rendered conditionally so older
  firmware (or non-ESP32 builds) that omit the field degrade
  gracefully.
  CPUUsageTracker.cpp is added to the esp32_base build_src_filter
  in platformio.ini.

examples/simple_repeater/MyMesh.h

@@ -11,6 +11,7 @@
   #include <LittleFS.h>
 #elif defined(ESP32)
   #include <SPIFFS.h>
+  #include "../../arch/esp32/CPUUsageTracker.h"
 #endif
 
 #ifdef WITH_RS232_BRIDGE
@@ -176,6 +177,9 @@ class MyMesh : public mesh::Mesh, public CommonCLICallbacks, public WebPanelComm
 #endif
 #if defined(ESP_PLATFORM) && WITH_WEB_PANEL
   WebService web;
+#endif
+#if defined(ESP32)
+  CPUUsageTracker _cpu_tracker;
 #endif
   StatsHistory _stats_history;
   struct {