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>

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; }
 };