fix: Defer low-power mode during section indexing and book loading

Prevent the device from dropping to 10MHz CPU during first-time chapter
indexing, cover prerendering, and other CPU-intensive reader operations.

Three issues addressed:
- ActivityWithSubactivity now delegates preventAutoSleep() and
  skipLoopDelay() to the active subactivity, so EpubReaderActivity's
  signal is visible through the ReaderActivity wrapper
- Added post-loop() re-check of preventAutoSleep() in main.cpp to
  catch activity transitions that happen mid-loop
- EpubReaderActivity uses both !section and a loadingSection flag to
  cover the full duration from activity entry through section file
  creation; TxtReaderActivity uses !initialized similarly

Also syncs HalPowerManager.cpp log messages with upstream PR #852.

Co-authored-by: Cursor <cursoragent@cursor.com>

lib/hal/HalPowerManager.cpp

@@ -17,14 +17,14 @@ void HalPowerManager::setPowerSaving(bool enabled) {
   if (enabled && !isLowPower) {
     LOG_DBG("PWR", "Going to low-power mode");
     if (!setCpuFrequencyMhz(LOW_POWER_FREQ)) {
-      LOG_ERR("PWR", "Failed to set low-power CPU frequency");
+      LOG_DBG("PWR", "Failed to set CPU frequency = %d MHz", LOW_POWER_FREQ);
       return;
     }
   }
   if (!enabled && isLowPower) {
     LOG_DBG("PWR", "Restoring normal CPU frequency");
     if (!setCpuFrequencyMhz(normalFreq)) {
-      LOG_ERR("PWR", "Failed to restore normal CPU frequency");
+      LOG_DBG("PWR", "Failed to set CPU frequency = %d MHz", normalFreq);
       return;
     }
   }

src/activities/ActivityWithSubactivity.h

@@ -14,4 +14,6 @@ class ActivityWithSubactivity : public Activity {
       : Activity(std::move(name), renderer, mappedInput) {}
   void loop() override;
   void onExit() override;
+  bool preventAutoSleep() override { return subActivity && subActivity->preventAutoSleep(); }
+  bool skipLoopDelay() override { return subActivity && subActivity->skipLoopDelay(); }
 };

src/activities/reader/EpubReaderActivity.cpp

@@ -861,6 +861,8 @@ void EpubReaderActivity::renderScreen() {
   }
 
   if (!section) {
+    loadingSection = true;
+
     const auto filepath = epub->getSpineItem(currentSpineIndex).href;
     LOG_DBG("ERS", "Loading file: %s, index: %d", filepath.c_str(), currentSpineIndex);
     section = std::unique_ptr<Section>(new Section(epub, currentSpineIndex, renderer));
@@ -880,6 +882,7 @@ void EpubReaderActivity::renderScreen() {
                                       viewportHeight, SETTINGS.hyphenationEnabled, SETTINGS.embeddedStyle, popupFn)) {
         LOG_ERR("ERS", "Failed to persist page data to SD");
         section.reset();
+        loadingSection = false;
         return;
       }
     } else {
@@ -912,6 +915,8 @@ void EpubReaderActivity::renderScreen() {
       section->currentPage = newPage;
       pendingPercentJump = false;
     }
+
+    loadingSection = false;
   }
 
   renderer.clearScreen();

src/activities/reader/EpubReaderActivity.h

@@ -29,6 +29,7 @@ class EpubReaderActivity final : public ActivityWithSubactivity {
   bool pendingSubactivityExit = false;  // Defer subactivity exit to avoid use-after-free
   bool pendingGoHome = false;           // Defer go home to avoid race condition with display task
   bool skipNextButtonCheck = false;     // Skip button processing for one frame after subactivity exit
+  volatile bool loadingSection = false;  // True during the entire !section block (read from main loop)
   const std::function<void()> onGoBack;
   const std::function<void()> onGoHome;
 
@@ -55,4 +56,10 @@ class EpubReaderActivity final : public ActivityWithSubactivity {
   void onEnter() override;
   void onExit() override;
   void loop() override;
+  // Defer low-power mode and auto-sleep while a section is loading/building.
+  // !section covers the period before the Section object is created (including
+  // cover prerendering in onEnter). loadingSection covers the full !section block
+  // in renderScreen (including createSectionFile), during which section is non-null
+  // but the section file is still being built.
+  bool preventAutoSleep() override { return !section || loadingSection; }
 };

src/activities/reader/TxtReaderActivity.h

@@ -57,4 +57,7 @@ class TxtReaderActivity final : public ActivityWithSubactivity {
   void onEnter() override;
   void onExit() override;
   void loop() override;
+  // Defer low-power mode and auto-sleep while the reader is initializing
+  // (cover prerendering, page index building on first open).
+  bool preventAutoSleep() override { return !initialized; }
 };

src/main.cpp

@@ -440,6 +440,13 @@ void loop() {
     }
   }
 
+  // Re-check preventAutoSleep: the activity may have changed during loop() above
+  // (e.g., HomeActivity transitioned to EpubReaderActivity with pending section work).
+  if (currentActivity && currentActivity->preventAutoSleep()) {
+    lastActivityTime = millis();
+    powerManager.setPowerSaving(false);
+  }
+
   // Add delay at the end of the loop to prevent tight spinning
   // When an activity requests skip loop delay (e.g., webserver running), use yield() for faster response
   // Otherwise, use longer delay to save power