Add framebuffer lending for memory-intensive builds

Temporarily release framebuffer memory during section pagination to reduce heap pressure. The framebuffer can be released before memory-intensive operations and restored afterward, allowing BLE stack startup threshold to be lowered from 80KB to 70KB. Implements RAII-style FrameBufferBuildLoan class to ensure proper cleanup and automatic restart on restoration failure.

lib/GfxRenderer/FontCacheManager.cpp

@@ -62,7 +62,14 @@ void FontCacheManager::resetStats() {
 bool FontCacheManager::isScanning() const { return scanMode_ == ScanMode::Scanning; }
 
 void FontCacheManager::recordText(const char* text, int fontId, EpdFontFamily::Style style) {
-  scanText_ += text;
+  if (!text) return;
+  const size_t remaining = (scanTextLen_ < SCAN_TEXT_CAPACITY - 1) ? (SCAN_TEXT_CAPACITY - 1 - scanTextLen_) : 0;
+  if (remaining > 0) {
+    const size_t textLen = strnlen(text, remaining);
+    memcpy(scanText_ + scanTextLen_, text, textLen);
+    scanTextLen_ += textLen;
+    scanText_[scanTextLen_] = '\0';
+  }
   if (scanFontId_ < 0) scanFontId_ = fontId;
   const uint8_t baseStyle = static_cast<uint8_t>(style) & 0x03;
   const unsigned char* p = reinterpret_cast<const unsigned char*>(text);
@@ -80,15 +87,15 @@ FontCacheManager::PrewarmScope::PrewarmScope(FontCacheManager& manager) : manage
   manager_->scanMode_ = ScanMode::Scanning;
   manager_->clearCache();
   manager_->resetStats();
-  manager_->scanText_.clear();
-  manager_->scanText_.reserve(2048);  // Pre-allocate to avoid heap fragmentation from repeated concat
+  manager_->scanTextLen_ = 0;
+  manager_->scanText_[0] = '\0';
   memset(manager_->scanStyleCounts_, 0, sizeof(manager_->scanStyleCounts_));
   manager_->scanFontId_ = -1;
 }
 
 void FontCacheManager::PrewarmScope::endScanAndPrewarm() {
   manager_->scanMode_ = ScanMode::None;
-  if (manager_->scanText_.empty()) return;
+  if (manager_->scanTextLen_ == 0) return;
 
   // Build style bitmask from all styles that appeared during the scan
   uint8_t styleMask = 0;
@@ -97,11 +104,10 @@ void FontCacheManager::PrewarmScope::endScanAndPrewarm() {
   }
   if (styleMask == 0) styleMask = 1;  // default to regular
 
-  manager_->prewarmCache(manager_->scanFontId_, manager_->scanText_.c_str(), styleMask);
+  manager_->prewarmCache(manager_->scanFontId_, manager_->scanText_, styleMask);
 
-  // Free scan string memory
-  manager_->scanText_.clear();
-  manager_->scanText_.shrink_to_fit();
+  manager_->scanTextLen_ = 0;
+  manager_->scanText_[0] = '\0';
 }
 
 FontCacheManager::PrewarmScope::~PrewarmScope() {

lib/GfxRenderer/FontCacheManager.h

@@ -2,9 +2,9 @@
 
 #include <EpdFontFamily.h>
 
+#include <cstddef>
 #include <cstdint>
 #include <map>
-#include <string>
 
 class FontDecompressor;
 class SdCardFont;
@@ -51,7 +51,9 @@ class FontCacheManager {
 
   enum class ScanMode : uint8_t { None, Scanning };
   ScanMode scanMode_ = ScanMode::None;
-  std::string scanText_;
+  static constexpr size_t SCAN_TEXT_CAPACITY = 2048;
+  char scanText_[SCAN_TEXT_CAPACITY] = {};
+  size_t scanTextLen_ = 0;
   uint32_t scanStyleCounts_[4] = {};
   int scanFontId_ = -1;
 };

lib/GfxRenderer/GfxRenderer.cpp

@@ -91,6 +91,20 @@ void GfxRenderer::begin() {
   bwBufferChunks.assign((frameBufferSize + BW_BUFFER_CHUNK_SIZE - 1) / BW_BUFFER_CHUNK_SIZE, nullptr);
 }
 
+void GfxRenderer::releaseFrameBufferForBuild() {
+  display.releaseFrameBuffers();
+  frameBuffer = nullptr;
+}
+
+bool GfxRenderer::restoreFrameBufferAfterBuild() {
+  if (!display.reallocFrameBuffers()) {
+    LOG_ERR("GFX", "Framebuffer realloc failed after build");
+    return false;
+  }
+  frameBuffer = display.getFrameBuffer();
+  return frameBuffer != nullptr;
+}
+
 bool GfxRenderer::isFontCacheScanning() const { return fontCacheManager_ && fontCacheManager_->isScanning(); }
 
 void GfxRenderer::insertFont(const int fontId, EpdFontFamily font) {

lib/GfxRenderer/GfxRenderer.h

@@ -250,6 +250,13 @@ class GfxRenderer {
   // Font helpers
   const uint8_t* getGlyphBitmap(const EpdFontData* fontData, const EpdGlyph* glyph) const;
 
+  // Lend the framebuffer to memory-hungry phases such as section pagination.
+  // Nothing may draw/display while it is released. restore returns the buffer
+  // white, so callers must redraw the full screen afterward.
+  void releaseFrameBufferForBuild();
+  bool restoreFrameBufferAfterBuild();
+  bool hasFrameBuffer() const { return frameBuffer != nullptr; }
+
   // Low level functions
   uint8_t* getFrameBuffer() const;
   size_t getBufferSize() const;

lib/hal/HalDisplay.cpp

@@ -77,6 +77,10 @@ void HalDisplay::deepSleep() { einkDisplay.deepSleep(); }
 
 uint8_t* HalDisplay::getFrameBuffer() const { return einkDisplay.getFrameBuffer(); }
 
+void HalDisplay::releaseFrameBuffers() { einkDisplay.releaseBuffers(); }
+
+bool HalDisplay::reallocFrameBuffers() { return einkDisplay.reallocBuffers(); }
+
 void HalDisplay::copyGrayscaleBuffers(const uint8_t* lsbBuffer, const uint8_t* msbBuffer) {
   einkDisplay.copyGrayscaleBuffers(lsbBuffer, msbBuffer);
 }

lib/hal/HalDisplay.h

@@ -47,6 +47,12 @@ class HalDisplay {
   // Access to frame buffer
   uint8_t* getFrameBuffer() const;
 
+  // Lend the framebuffer's RAM to a memory-hungry phase. No display calls may
+  // run between release and a successful realloc; buffers come back white, so
+  // callers must redraw the full screen.
+  void releaseFrameBuffers();
+  bool reallocFrameBuffers();
+
   // X3 grayscale preconditioning (OEM "AA-pre-BW(mid)" settle pass), windowed
   // to the gray region in physical panel coordinates (no-arg = full frame).
   // Call after the BW base frame is displayed and before the grayscale planes