fix(xtc): address PR review feedback for grayscale rendering

- Use FsHelpers::openFileForRead for consistency (XtcParser.cpp)
- Simplify file extension checking logic (FileSelectionActivity.cpp)
- Fix grayscale rendering bugs (XtcReaderActivity.cpp):
  - Use drawPixel(x, y, false) for LSB/MSB passes (LUT: 0=apply effect)
  - Add cleanupGrayscaleWithFrameBuffer() after re-rendering BW
  - Use conditional HALF_REFRESH based on pagesUntilFullRefresh
  - Decrement refresh counter instead of resetting
- Add cleanupGrayscaleWithFrameBuffer() helper (GfxRenderer)

lib/GfxRenderer/GfxRenderer.cpp

@@ -369,6 +369,17 @@ void GfxRenderer::restoreBwBuffer() {
   Serial.printf("[%lu] [GFX] Restored and freed BW buffer chunks\n", millis());
 }
 
+/**
+ * Cleanup grayscale buffers using the current frame buffer.
+ * Use this when BW buffer was re-rendered instead of stored/restored.
+ */
+void GfxRenderer::cleanupGrayscaleWithFrameBuffer() const {
+  uint8_t* frameBuffer = einkDisplay.getFrameBuffer();
+  if (frameBuffer) {
+    einkDisplay.cleanupGrayscaleBuffers(frameBuffer);
+  }
+}
+
 void GfxRenderer::renderChar(const EpdFontFamily& fontFamily, const uint32_t cp, int* x, const int* y,
                              const bool pixelState, const EpdFontStyle style) const {
   const EpdGlyph* glyph = fontFamily.getGlyph(cp, style);

lib/GfxRenderer/GfxRenderer.h

@@ -67,6 +67,7 @@ class GfxRenderer {
   void displayGrayBuffer() const;
   bool storeBwBuffer();  // Returns true if buffer was stored successfully
   void restoreBwBuffer();
+  void cleanupGrayscaleWithFrameBuffer() const;
 
   // Low level functions
   uint8_t* getFrameBuffer() const;

lib/Xtc/Xtc/XtcParser.cpp

@@ -7,6 +7,7 @@
 
 #include "XtcParser.h"
 
+#include <FsHelpers.h>
 #include <HardwareSerial.h>
 
 #include <cstring>
@@ -31,9 +32,7 @@ XtcError XtcParser::open(const char* filepath) {
   }
 
   // Open file
-  m_file = SD.open(filepath, FILE_READ);
-  if (!m_file) {
-    Serial.printf("[%lu] [XTC] Failed to open file: %s\n", millis(), filepath);
+  if (!FsHelpers::openFileForRead("XTC", filepath, m_file)) {
     m_lastError = XtcError::FILE_NOT_FOUND;
     return m_lastError;
   }

src/activities/reader/FileSelectionActivity.cpp

@@ -40,18 +40,11 @@ void FileSelectionActivity::loadFiles() {
 
     if (file.isDirectory()) {
       files.emplace_back(filename + "/");
-    } else if (filename.length() >= 5 && filename.substr(filename.length() - 5) == ".epub") {
-      files.emplace_back(filename);
-    } else if (filename.length() >= 4) {
-      // Check for XTC format extensions (.xtc, .xtch)
-      std::string ext4 = filename.substr(filename.length() - 4);
-      if (ext4 == ".xtc") {
+    } else {
+      std::string ext4 = filename.length() >= 4 ? filename.substr(filename.length() - 4) : "";
+      std::string ext5 = filename.length() >= 5 ? filename.substr(filename.length() - 5) : "";
+      if (ext5 == ".epub" || ext5 == ".xtch" || ext4 == ".xtc") {
         files.emplace_back(filename);
-      } else if (filename.length() >= 5) {
-        std::string ext5 = filename.substr(filename.length() - 5);
-        if (ext5 == ".xtch") {
-          files.emplace_back(filename);
-        }
       }
     }
     file.close();

src/activities/reader/XtcReaderActivity.cpp

@@ -236,29 +236,33 @@ void XtcReaderActivity::renderPage() {
       }
     }
 
-    // Display BW first with half refresh (clean base for grayscale overlay)
-    renderer.displayBuffer(EInkDisplay::HALF_REFRESH);
+    // Display BW with conditional refresh based on pagesUntilFullRefresh
+    if (pagesUntilFullRefresh <= 1) {
+      renderer.displayBuffer(EInkDisplay::HALF_REFRESH);
+    } else {
+      renderer.displayBuffer();
+    }
 
     // Pass 2: LSB buffer - mark DARK gray only (XTH value 1)
-    // README: "mark the **dark** grays pixels with `1`"
+    // In LUT: 0 bit = apply gray effect, 1 bit = untouched
     renderer.clearScreen(0x00);
     for (uint16_t y = 0; y < pageHeight; y++) {
       for (uint16_t x = 0; x < pageWidth; x++) {
         if (getPixelValue(x, y) == 1) {  // Dark grey only
-          renderer.drawPixel(x, y, true);
+          renderer.drawPixel(x, y, false);
         }
       }
     }
     renderer.copyGrayscaleLsbBuffers();
 
     // Pass 3: MSB buffer - mark LIGHT AND DARK gray (XTH value 1 or 2)
-    // README: "mark the **light and dark** grays pixels with `1`"
+    // In LUT: 0 bit = apply gray effect, 1 bit = untouched
     renderer.clearScreen(0x00);
     for (uint16_t y = 0; y < pageHeight; y++) {
       for (uint16_t x = 0; x < pageWidth; x++) {
         const uint8_t pv = getPixelValue(x, y);
         if (pv == 1 || pv == 2) {  // Dark grey or Light grey
-          renderer.drawPixel(x, y, true);
+          renderer.drawPixel(x, y, false);
         }
       }
     }
@@ -277,8 +281,11 @@ void XtcReaderActivity::renderPage() {
       }
     }
 
-    // Reset refresh counter (grayscale display is a full refresh)
-    pagesUntilFullRefresh = pagesPerRefresh;
+    // Cleanup grayscale buffers with current frame buffer
+    renderer.cleanupGrayscaleWithFrameBuffer();
+
+    // Decrement refresh counter
+    pagesUntilFullRefresh--;
 
     free(pageBuffer);