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/types/src/display.rs

@@ -0,0 +1,36 @@
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct Color(u32);
+
+impl Color {
+    pub const fn from_raw(v: u32) -> Self { Self(v) }
+    pub const fn raw(&self) -> u32 { self.0 }
+
+    pub const fn rgb(r: u8, g: u8, b: u8) -> Self {
+        Self(((r as u32) << 16) | ((g as u32) << 8) | (b as u32) | 0xFF000000)
+    }
+
+    pub const fn rgba(r: u8, g: u8, b: u8, a: u8) -> Self {
+        Self(((a as u32) << 24) | ((r as u32) << 16) | ((g as u32) << 8) | (b as u32))
+    }
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
+pub struct ScreenCoord(u16);
+
+impl ScreenCoord {
+    pub const fn new(v: u16) -> Self { Self(v) }
+    pub const fn raw(&self) -> u16 { self.0 }
+}
+
+impl core::ops::Add<Dimension> for ScreenCoord {
+    type Output = Self;
+    fn add(self, rhs: Dimension) -> Self { Self(self.0 + rhs.0) }
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
+pub struct Dimension(u16);
+
+impl Dimension {
+    pub const fn new(v: u16) -> Self { Self(v) }
+    pub const fn raw(&self) -> u16 { self.0 }
+}

crates/types/src/lib.rs

@@ -2,6 +2,7 @@
 
 pub mod address;
 pub mod bounded;
+pub mod display;
 pub mod generation;
 pub mod id;
 pub mod macros;

crates/types/src/permission.rs

@@ -44,4 +44,8 @@ impl PermissionSet {
     pub fn from_bits(bits: u8) -> Self {
         PermissionSet(bits)
     }
+
+    pub fn bits(&self) -> u8 {
+        self.0
+    }
 }