console: page-wrap instead of scrolling — never read the framebuffer

The scroll path copied every pixel row up by one glyph height, READING
video memory — and VRAM reads are uncached-slow on real hardware.
Measured on the 16-core PC: ~90 seconds to bring the cores online,
almost entirely boot-transcript lines each paying a whole-screen scroll
copy. (QEMU never shows this: its 'VRAM' is host RAM.)

When the screen fills, the console now clears and restarts at the top —
writes only, once per screenful. The transcript reads the same as it
streams; only the scrollback illusion is gone, which a boot console
never needed.

system/kernel/console.zig

@@ -138,14 +138,16 @@ pub const Console = struct {
         }
     }
 
-    /// Shift the visible text up one glyph row and clear the freed bottom row,
-    /// leaving the cursor on that now-blank last line.
+    /// The screen is full: start a fresh page at the top. NEVER scroll by
+    /// copying pixel rows — that READS the framebuffer, and VRAM reads are
+    /// uncached-slow on real hardware (measured: 16-core bring-up took ~90 s
+    /// purely from boot lines each paying a whole-screen scroll copy). A page
+    /// clear is writes only, and only once per screenful.
     fn scroll(self: *Console) void {
         const visible = self.rows * glyph_h;
         var y: u32 = 0;
-        while (y + glyph_h < visible) : (y += 1) self.copyRow(y, y + glyph_h);
         while (y < visible) : (y += 1) self.fillRow(y, self.bg);
-        self.row = self.rows - 1;
+        self.row = 0;
     }
 
     inline fn rowPtr(self: *Console, y: u32) [*]volatile u32 {
@@ -163,10 +165,4 @@ pub const Console = struct {
         while (x < self.fb.width) : (x += 1) row[x] = color;
     }
 
-    fn copyRow(self: *Console, destination_y: u32, source_y: u32) void {
-        const destination = self.rowPtr(destination_y);
-        const source = self.rowPtr(source_y);
-        var x: u32 = 0;
-        while (x < self.fb.width) : (x += 1) destination[x] = source[x];
-    }
 };