fix: handle fallbacks for advance table and prewarm (#1929)

## Summary
- Fixes #1928 by having the prewarm and advance table functions resolve
fallback styles
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### AI Usage
Did you use AI tools to help write this code?  YES - Codex

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Co-authored-by: Uri Tauber <uritaube@gmail.com>

lib/EpdFont/SdCardFont.h

@@ -71,6 +71,13 @@ class SdCardFont {
   // Returns true if the given style is present in this font file.
   bool hasStyle(uint8_t style) const;
 
+  // Resolve requested style bits to the closest present style.
+  uint8_t resolveStyle(uint8_t style) const;
+
+  // Resolve every requested style bit through fallback and return the actual
+  // styles that need cache/advance preparation.
+  uint8_t resolveStyleMask(uint8_t styleMask) const;
+
   // Number of styles present in this font file.
   uint8_t styleCount() const { return styleCount_; }