fix: Prepare SD card font caches from txt reader (#1973)

## Summary

SD card font fixes:
- `TxtReaderActivity` needs to call `renderer.ensureSdCardFontReady` to
build the advance lookup table to support rendering with SD card fonts.
This revealed that `TxtReaderActivity` was inconsistently performing
layout with `getTextWidth`, when the renderer actually uses
`getTextAdvanceX`, which can lead to minor inconsistencies in alignment.
- Avoid allocating one big `allText` string in
`ParsedText::layoutAndExtractLines`. Instead, pass the vector of word
strings directly to `SdCardFont::buildAdvanceTable`, where the algorithm
just needs to iterate codepoints anyway.

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Co-authored-by: Justin Mitchell <justin@jmitch.com>

lib/Epub/Epub/ParsedText.cpp

@@ -255,20 +255,6 @@ void ParsedText::layoutAndExtractLines(const GfxRenderer& renderer, const int fo
   // (advanceX only, no bitmaps) for all unique codepoints in this paragraph so
   // that calculateWordWidths() can measure text without on-demand SD I/O.
   if (renderer.isSdCardFont(fontId)) {
-    // Reserve upfront so the joined text allocates exactly once. Without this,
-    // paragraphs with many words trigger a chain of vector-like reallocations
-    // inside std::string during layout — visible in prewarm timings for SD fonts.
-    size_t totalSize = hyphenationEnabled ? 1 : 0;
-    if (!words.empty()) totalSize += words.size() - 1;  // inter-word spaces
-    for (const auto& w : words) totalSize += w.size();
-    std::string allText;
-    allText.reserve(totalSize);
-    for (size_t i = 0; i < words.size(); i++) {
-      if (i > 0) allText += ' ';
-      allText += words[i];
-    }
-    if (hyphenationEnabled) allText += '-';
-
     // Style mask: only ask the SD font to load advances for styles actually
     // used in this paragraph. Style index is the low two bits (regular/bold/
     // italic/bold-italic); the underline bit is irrelevant to advance metrics.
@@ -277,7 +263,7 @@ void ParsedText::layoutAndExtractLines(const GfxRenderer& renderer, const int fo
       styleMask |= static_cast<uint8_t>(1u << (static_cast<uint8_t>(s) & 0x03));
     }
     if (styleMask == 0) styleMask = 0x01;  // defensive: regular only
-    renderer.ensureSdCardFontReady(fontId, allText.c_str(), styleMask);
+    renderer.ensureSdCardFontReady(fontId, words, hyphenationEnabled, styleMask);
   }
 
   const int pageWidth = viewportWidth;