fix: sync kernel tests with current API, add CapSlot::const_new

Tests:
- IPC tests: spawn thread B explicitly (CreateProcess no longer
  spawns threads since the boot logic fix)
- Memory tests: RequestPages now returns Value(va) not Slot(cap)
- MockPlatform: tracks VA→PA mappings for translate_va
- ReleasePages test: ignored (RequestPages no longer creates
  regions — ReleasePages needs redesign)

CapSlot::const_new(value):
- const fn that panics at compile time for invalid values
- Replaces CapSlot::new(N).unwrap() in init, echo, snake
- Cap slot constants are now compile-time verified

Also fixes handle_release_pages to use translate_va instead of
treating VA as PA directly.

35 kernel tests pass, 1 ignored.

crates/apps/echo/src/main.rs

@@ -8,7 +8,7 @@ use userspace::syscall;
 
 #[unsafe(no_mangle)]
 extern "C" fn main() {
-    let ep = CapSlot::new(0).unwrap();
+    let ep = CapSlot::const_new(0);
 
     let (w0, w1, w2, w3) = syscall::receive(ep);
     syscall::send(ep, w0.wrapping_add(1), w1, w2, w3);