refactor: apply newtypes throughout userspace API and display types

Every syscall stub now uses proper types from types/ — CapSlot,
EntryPoint, StackPointer, VirtualAddress, PageCount, ProcessId,
PermissionSet, Ticks. The compiler prevents swapping entry/stack,
passing a ProcessId where a CapSlot is expected, or using an
address as a page count.

New display types in types/display.rs:
  - Color(u32) with rgb() constructor
  - ScreenCoord(u16) for x/y positions
  - Dimension(u16) for width/height

Updated throughout:
  - userspace/syscall.rs: all params typed, returns Option instead
    of sentinel usize::MAX
  - server-framework: endpoint param is CapSlot
  - port-framebuffer: trait uses Color, ScreenCoord, Dimension
  - hw-virtio: DmaAddrs uses PhysicalAddress/VirtualAddress,
    GPU commands use Dimension
  - All servers, drivers, bins, apps updated for new signatures
  - permission-manager: process field uses ProcessId
  - PermissionSet: added bits() accessor

89 tests pass, full QEMU boot verified.

crates/drivers/framebuffer/mock/Cargo.toml

@@ -5,3 +5,4 @@ edition = "2024"
 
 [dependencies]
 port-framebuffer = { workspace = true }
+types = { workspace = true }

crates/drivers/framebuffer/mock/src/lib.rs

@@ -1,21 +1,22 @@
 #![no_std]
 
 use port_framebuffer::Framebuffer;
+use types::display::{Color, Dimension, ScreenCoord};
 
 pub struct MockFramebuffer {
-    width: u16,
-    height: u16,
-    pixels: [(u16, u16, u32); 4096],
+    width: Dimension,
+    height: Dimension,
+    pixels: [(ScreenCoord, ScreenCoord, Color); 4096],
     pixel_count: usize,
     flushed: bool,
 }
 
 impl MockFramebuffer {
-    pub fn new(width: u16, height: u16) -> Self {
+    pub fn new(width: Dimension, height: Dimension) -> Self {
         Self {
             width,
             height,
-            pixels: [(0, 0, 0); 4096],
+            pixels: [(ScreenCoord::new(0), ScreenCoord::new(0), Color::from_raw(0)); 4096],
             pixel_count: 0,
             flushed: false,
         }
@@ -29,31 +30,31 @@ impl MockFramebuffer {
         self.flushed
     }
 
-    pub fn pixel_at(&self, index: usize) -> (u16, u16, u32) {
+    pub fn pixel_at(&self, index: usize) -> (ScreenCoord, ScreenCoord, Color) {
         self.pixels[index]
     }
 }
 
 impl Framebuffer for MockFramebuffer {
-    fn width(&self) -> u16 { self.width }
-    fn height(&self) -> u16 { self.height }
+    fn width(&self) -> Dimension { self.width }
+    fn height(&self) -> Dimension { self.height }
 
-    fn write_pixel(&mut self, x: u16, y: u16, color: u32) {
+    fn write_pixel(&mut self, x: ScreenCoord, y: ScreenCoord, color: Color) {
         if self.pixel_count < self.pixels.len() {
             self.pixels[self.pixel_count] = (x, y, color);
             self.pixel_count += 1;
         }
     }
 
-    fn fill_rect(&mut self, x: u16, y: u16, w: u16, h: u16, color: u32) {
-        for dy in 0..h {
-            for dx in 0..w {
-                self.write_pixel(x + dx, y + dy, color);
+    fn fill_rect(&mut self, x: ScreenCoord, y: ScreenCoord, w: Dimension, h: Dimension, color: Color) {
+        for dy in 0..h.raw() {
+            for dx in 0..w.raw() {
+                self.write_pixel(ScreenCoord::new(x.raw() + dx), ScreenCoord::new(y.raw() + dy), color);
             }
         }
     }
 
-    fn clear(&mut self, _color: u32) {
+    fn clear(&mut self, _color: Color) {
         self.pixel_count = 0;
     }