fix: Use differential rounding for consistent inter-glyph spacing (#1413)

## Summary

**What is the goal of this PR?**

A tweak to the fixed-point x-advance and kerning calculations to ensure
that the spacing between any two glyphs is always calculated
consistently.

I noticed that sometimes I'd see common character pairs like "oo" more
than once on a page, and the distance between the two snapped to
different pixels depending on the running accumulated error for the line
of text.

This change uses a differential rounding approach where each glyph's
x-advance plus the kerning relative to the next glyph are combined in
fixed-point precision, then snapped to a pixel to draw the next glyph.
This results in a consistent inter-glyph spacing any time the same two
glyphs show up adjacent to each other, regardless of the accumulated
error across the line.

---

### AI Usage

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Did you use AI tools to help write this code? _**PARTIALLY**_

lib/EpdFont/EpdFont.cpp

@@ -15,10 +15,10 @@ void EpdFont::getTextBounds(const char* string, const int startX, const int star
     return;
   }
 
-  int32_t cursorXFP = fp4::fromPixel(startX);  // 12.4 fixed-point accumulator
   int lastBaseX = startX;
   int lastBaseAdvanceFP = 0;  // 12.4 fixed-point
   int lastBaseTop = 0;
+  int32_t prevAdvanceFP = 0;  // 12.4 fixed-point: prev glyph's advance + next kern for snap
   constexpr int MIN_COMBINING_GAP_PX = 1;
   uint32_t cp;
   uint32_t prevCp = 0;
@@ -31,7 +31,9 @@ void EpdFont::getTextBounds(const char* string, const int startX, const int star
 
     const EpdGlyph* glyph = getGlyph(cp);
     if (!glyph) {
+      lastBaseX += fp4::toPixel(prevAdvanceFP);  // flush pending advance before resetting
       prevCp = 0;
+      prevAdvanceFP = 0;
       continue;
     }
 
@@ -44,11 +46,11 @@ void EpdFont::getTextBounds(const char* string, const int startX, const int star
     }
 
     if (!isCombining && prevCp != 0) {
-      cursorXFP += getKerning(prevCp, cp);  // 4.4 fixed-point kern
+      const auto kernFP = getKerning(prevCp, cp);  // 4.4 fixed-point kern
+      lastBaseX += fp4::toPixel(prevAdvanceFP + kernFP);
     }
 
-    const int cursorXPixels = fp4::toPixel(cursorXFP);  // snap 12.4 fixed-point to nearest pixel
-    const int glyphBaseX = isCombining ? (lastBaseX + fp4::toPixel(lastBaseAdvanceFP / 2)) : cursorXPixels;
+    const int glyphBaseX = isCombining ? (lastBaseX + fp4::toPixel(lastBaseAdvanceFP / 2)) : lastBaseX;
     const int glyphBaseY = startY - raiseBy;
 
     *minX = std::min(*minX, glyphBaseX + glyph->left);
@@ -57,10 +59,9 @@ void EpdFont::getTextBounds(const char* string, const int startX, const int star
     *maxY = std::max(*maxY, glyphBaseY + glyph->top);
 
     if (!isCombining) {
-      lastBaseX = cursorXPixels;
       lastBaseAdvanceFP = glyph->advanceX;  // 12.4 fixed-point
       lastBaseTop = glyph->top;
-      cursorXFP += glyph->advanceX;  // 12.4 fixed-point advance
+      prevAdvanceFP = lastBaseAdvanceFP;
       prevCp = cp;
     }
   }