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.

Cargo.lock

@@ -57,6 +57,7 @@ dependencies = [
  "display-server",
  "driver-framebuffer-virtio",
  "server-framework",
+ "types",
  "userspace",
  "userspace-rt",
 ]
@@ -84,6 +85,7 @@ version = "0.1.0"
 dependencies = [
  "hw-virtio",
  "port-block-device",
+ "types",
 ]
 
 [[package]]
@@ -91,6 +93,7 @@ name = "driver-framebuffer-mock"
 version = "0.1.0"
 dependencies = [
  "port-framebuffer",
+ "types",
 ]
 
 [[package]]
@@ -99,12 +102,14 @@ version = "0.1.0"
 dependencies = [
  "hw-virtio",
  "port-framebuffer",
+ "types",
 ]
 
 [[package]]
 name = "echo"
 version = "0.1.0"
 dependencies = [
+ "types",
  "userspace",
  "userspace-rt",
 ]
@@ -131,6 +136,9 @@ dependencies = [
 [[package]]
 name = "hw-virtio"
 version = "0.1.0"
+dependencies = [
+ "types",
+]
 
 [[package]]
 name = "init"
@@ -217,6 +225,9 @@ version = "0.1.0"
 [[package]]
 name = "port-framebuffer"
 version = "0.1.0"
+dependencies = [
+ "types",
+]
 
 [[package]]
 name = "port-input-device"