perf(ui): O(n) FullscreenMsgView wrap via getCodepointWidth virtual

The wrap loop used to memcpy a growing substring and call getTextWidth
on it for every appended codepoint — O(n²) UTF-8 decoding with Lemon
font, ~5-20 ms on a full e-ink redraw of a long message.

Add DisplayDriver::getCodepointWidth(cp) (default: build a UTF-8 buffer
and forward to getTextWidth) plus DisplayDriver::decodeCodepoint() as
the shared UTF-8 walker. Override getCodepointWidth in GxEPDDisplay
(direct lemonXAdvance for Lemon size 1) and SH1106Display (lemonXAdvance
for Lemon, fixed 6*sz for built-in font). FullscreenMsgView now walks
codepoints once and sums widths incrementally.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/helpers/ui/DisplayDriver.h

@@ -160,7 +160,48 @@ public:
   virtual void translateUTF8ToBlocks(char* dest, const char* src, size_t dest_size) {
     translateUTF8Static(dest, src, dest_size);
   }
-  
+
+  // Advance a UTF-8 pointer by one codepoint, returning the decoded value.
+  // Invalid sequences return 0xFFFD and consume trailing continuation bytes.
+  static uint32_t decodeCodepoint(const uint8_t*& p) {
+    uint8_t c = *p++;
+    if (c < 0x80) return c;
+    if ((c & 0xE0) == 0xC0) {
+      uint32_t cp = c & 0x1F;
+      if (*p) cp = (cp << 6) | (*p++ & 0x3F);
+      return cp;
+    }
+    if ((c & 0xF0) == 0xE0) {
+      uint32_t cp = c & 0x0F;
+      if (*p) cp = (cp << 6) | (*p++ & 0x3F);
+      if (*p) cp = (cp << 6) | (*p++ & 0x3F);
+      return cp;
+    }
+    if ((c & 0xF8) == 0xF0) {
+      uint32_t cp = c & 0x07;
+      if (*p) cp = (cp << 6) | (*p++ & 0x3F);
+      if (*p) cp = (cp << 6) | (*p++ & 0x3F);
+      if (*p) cp = (cp << 6) | (*p++ & 0x3F);
+      return cp;
+    }
+    while (*p && (*p & 0xC0) == 0x80) p++;
+    return 0xFFFD;
+  }
+
+  // Width of a single codepoint in pixels. Default: fall back to getTextWidth
+  // on a one-codepoint UTF-8 string. Drivers can override for O(1) lookup.
+  virtual uint16_t getCodepointWidth(uint32_t cp) {
+    char buf[5];
+    int n = 0;
+    if (cp < 0x80) { buf[n++] = (char)cp; }
+    else if (cp < 0x800) { buf[n++] = 0xC0 | (cp >> 6); buf[n++] = 0x80 | (cp & 0x3F); }
+    else if (cp < 0x10000) { buf[n++] = 0xE0 | (cp >> 12); buf[n++] = 0x80 | ((cp >> 6) & 0x3F); buf[n++] = 0x80 | (cp & 0x3F); }
+    else { buf[n++] = 0xF0 | (cp >> 18); buf[n++] = 0x80 | ((cp >> 12) & 0x3F); buf[n++] = 0x80 | ((cp >> 6) & 0x3F); buf[n++] = 0x80 | (cp & 0x3F); }
+    buf[n] = '\0';
+    return getTextWidth(buf);
+  }
+
+
   // draw text with ellipsis if it exceeds max_width
   virtual void drawTextEllipsized(int x, int y, int max_width, const char* str) {
     char temp_str[256];  // reasonable buffer size

src/helpers/ui/GxEPDDisplay.h

@@ -114,6 +114,10 @@ public:
   void drawRect(int x, int y, int w, int h) override;
   void drawXbm(int x, int y, const uint8_t* bits, int w, int h) override;
   uint16_t getTextWidth(const char* str) override;
+  uint16_t getCodepointWidth(uint32_t cp) override {
+    if (_use_lemon && _text_sz == 1) return lemonXAdvance(cp, scale());
+    return DisplayDriver::getCodepointWidth(cp);
+  }
   void setDisplayRotation(uint8_t rot) override;
   void setFullRefreshInterval(uint8_t n) override { _full_refresh_interval = n; _partial_count = 0; }
   void endFrame() override;

src/helpers/ui/SH1106Display.h

@@ -48,6 +48,10 @@ public:
   void drawRect(int x, int y, int w, int h) override;
   void drawXbm(int x, int y, const uint8_t *bits, int w, int h) override;
   uint16_t getTextWidth(const char *str) override;
+  uint16_t getCodepointWidth(uint32_t cp) override {
+    if (_use_lemon) return lemonXAdvance(cp);
+    return 6 * _text_sz;  // built-in 5x7 font: 6 px advance per glyph
+  }
   int getCharWidth() const override { return (_use_lemon ? 5 : 6) * _text_sz; }
   int getLineHeight() const override { return (_use_lemon ? 9 : 8) * _text_sz; }
   void setLemonFont(bool enabled) override { _use_lemon = enabled; _vw_dirty = true; }