feat(l1): Home pages slide sideways, the page pill glides along

DisplayDriver gains slideBegin()/slideCompose(): the driver keeps the frame
on screen and, in the next one, moves it out while the new rows follow.
SH1106 does it on its 1 KB buffer (+1 KB RAM for the copy), the sim on its
canvas; drivers that can't read their frame back keep cutting over (e-ink).

Home slides the content below the page-icon row in 220 ms with an ease-out
(the whole screen to and from the header-less clock), and the current-page
pill glides between icons.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/helpers/ui/DisplayDriver.h

@@ -533,6 +533,14 @@ public:
   virtual void setBrightness(uint8_t level) { }  // level 0-4 (min to max), no-op default
   virtual void setDisplayRotation(uint8_t rot) { }  // 0-3, no-op for fixed-orientation displays
   virtual void setFullRefreshInterval(uint8_t n) { }  // e-ink: do full refresh every n partial refreshes (0=never)
+  // Sideways slide between two frames, for screens that page left/right.
+  // slideBegin() keeps rows y0.. of the frame on screen now; call it before the
+  // next frame is drawn. slideCompose(dx), at the end of drawing that frame,
+  // moves the kept rows dx px left (right when dx < 0) and fills the gap with
+  // the new frame's rows sliding in behind them. A driver that can't read its
+  // frame back returns false and the screen just cuts over.
+  virtual bool slideBegin(int y0) { return false; }
+  virtual void slideCompose(int dx) { }
   virtual void endFrame() = 0;
 
   // Called from board setup to run safe background work during a blocking

src/helpers/ui/SH1106Display.cpp

@@ -1,4 +1,5 @@
 #include "SH1106Display.h"
+#include <string.h>
 #include <Adafruit_GrayOLED.h>
 #include "Adafruit_SH110X.h"
 #include "MiscFixedRenderer.h"
@@ -64,6 +65,36 @@ void SH1106Display::startFrame(Color bkg)
   display.cp437(true); // Use full 256 char 'Code Page 437' font
 }
 
+// The GFX buffer still holds the last frame sent until the next startFrame().
+bool SH1106Display::slideBegin(int y0)
+{
+  _slide_y0 = y0 < 0 ? 0 : (y0 > height() ? height() : y0);
+  memcpy(_slide_old, display.getBuffer(), sizeof(_slide_old));
+  return true;
+}
+
+// Buffer layout: one byte per column per 8-row page, bit n = row page*8+n.
+void SH1106Display::slideCompose(int dx)
+{
+  uint8_t* buf = display.getBuffer();
+  const int W = width();
+  int off = dx < 0 ? -dx : dx;
+  if (off > W) off = W;
+  uint8_t fresh[128];
+  for (int p = _slide_y0 / 8; p < height() / 8; p++) {
+    const uint8_t keep = (p == _slide_y0 / 8) ? (uint8_t)~(0xFF << (_slide_y0 & 7)) : 0;
+    uint8_t* cur = buf + p * W;
+    const uint8_t* old = _slide_old + p * W;
+    memcpy(fresh, cur, W);
+    for (int x = 0; x < W; x++) {
+      uint8_t v;
+      if (dx >= 0) v = x < W - off ? old[x + off] : fresh[x - (W - off)];
+      else         v = x >= off    ? old[x - off] : fresh[x + (W - off)];
+      cur[x] = (cur[x] & keep) | (v & ~keep);
+    }
+  }
+}
+
 void SH1106Display::setTextSize(int sz)
 {
   _text_sz = sz;

src/helpers/ui/SH1106Display.h

@@ -44,6 +44,9 @@ class SH1106Display : public DisplayDriver
   static const uint8_t BLOCKS = (128 / BLOCK_W) * (64 / 8);
   uint32_t _block_hash[BLOCKS] = {};
   bool     _force_redraw = true;
+  // slideBegin()'s copy of the frame on screen (only rows >= _slide_y0 are used).
+  uint8_t  _slide_old[128 * 64 / 8];
+  int      _slide_y0 = 0;
 
   bool i2c_probe(TwoWire &wire, uint8_t addr);
   // Thin wrapper over the shared misc-fixed renderer (MiscFixedRenderer.h), kept
@@ -61,6 +64,8 @@ public:
   void turnOff() override;
   void clear() override;
   void startFrame(Color bkg = DARK) override;
+  bool slideBegin(int y0) override;
+  void slideCompose(int dx) override;
   void setTextSize(int sz) override;
   void setColor(Color c) override;
   void setCursor(int x, int y) override;