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/kernel/src/syscall/memory.rs

@@ -92,7 +92,12 @@ impl<P: Platform> Kernel<P> {
         count: types::bounded::PageCount,
     ) -> SyscallOutcome {
         let pages = count.value() as usize;
-        let phys = PhysicalAddress::from_raw(addr.raw());
+        let phys = match self.platform.translate_va(pid, addr) {
+            Some(pa) => pa,
+            None => return SyscallOutcome::Complete(SyscallResult::Error(
+                domain::error::KernelError::OutOfMemory,
+            )),
+        };
 
         let region_id = match self.regions.find_by_base(pid, phys) {
             Some((rid, _)) => rid,