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/types/src/macros.rs

@@ -32,6 +32,11 @@ macro_rules! bounded {
                 }
             }
 
+            pub const fn const_new(value: $inner) -> Self {
+                assert!(value >= Self::MIN && value <= Self::MAX, "bounded value out of range");
+                Self(value)
+            }
+
             pub fn value(&self) -> $inner {
                 self.0
             }