fix: per-CPU idle context race condition + nightly-2026-04-11 pin

- Add try_idle_context() to ContextSwitchPercpu (switch.rs)
  Cross-CPU paths (steal_work, migrate_one_context) use try_idle_context()
  instead of idle_context() to avoid panic when APs haven't called
  context::init() yet. Returns Option<context::Arc> instead of panicking.
- Pin rust-toolchain.toml to nightly-2026-04-11
- Remove build artifacts (kernel, kernel.all, kernel.sym) from git tracking
- This fixes the boot panic that occurred during multi-CPU scheduling

.gitignore

@@ -1,3 +1,6 @@
 target
 /config.toml
 .gitlab-ci-local/
+kernel
+kernel.all
+kernel.sym

Makefile

@@ -31,8 +31,8 @@ TARGET_SPEC=$(RUST_TARGET_PATH)/$(ARCH)-unknown-kernel.json
 KERNEL_CARGO_FEATURES?=
 
 $(BUILD)/kernel.all: $(LD_SCRIPT) $(LOCKFILE) $(MANIFEST) $(TARGET_SPEC) $(shell find $(SOURCE) -name "*.rs" -type f)
-	cd $(SOURCE) && RUSTUP_TOOLCHAIN=nightly-2025-10-03 cargo rustc \
-		-Z build-std=core,alloc -Zbuild-std-features=compiler-builtins-mem \
+	cd $(SOURCE) && RUSTUP_TOOLCHAIN=nightly-2026-04-11 RUSTFLAGS="-Zunstable-options" cargo rustc \
+		-Z json-target-spec -Z build-std=core,alloc -Zbuild-std-features=compiler-builtins-mem \
 		--bin kernel \
 		--manifest-path "$(MANIFEST)" \
 		--target "$(TARGET_SPEC)" \

rust-toolchain.toml

@@ -1,3 +1,3 @@
 [toolchain]
-channel = "nightly-2025-10-03"
+channel = "nightly-2026-04-11"
 components = ["rust-src"]

src/context/switch.rs

@@ -399,7 +399,9 @@ fn steal_work(
             continue;
         };
 
-        let victim_idle = victim.switch_internals.idle_context();
+        let Some(victim_idle) = victim.switch_internals.try_idle_context() else {
+            continue;
+        };
         let mut victim_lock = SchedQueuesLock::new(&victim.sched);
         let victim_queues = unsafe { victim_lock.queues_mut() };
 
@@ -469,7 +471,9 @@ fn migrate_one_context(
         return false;
     };
 
-    let source_idle = source.switch_internals.idle_context();
+    let Some(source_idle) = source.switch_internals.try_idle_context() else {
+        return false;
+    };
     let moved = {
         let mut source_lock = SchedQueuesLock::new(&source.sched);
         let source_queues = unsafe { source_lock.queues_mut() };
@@ -1174,4 +1178,17 @@ impl ContextSwitchPercpu {
                 .expect("no idle context present"),
         )
     }
+
+    /// Retrieves the current idle context if it has been initialized.
+    ///
+    /// This is the fallible variant of [`idle_context`], intended for
+    /// cross-CPU paths (`steal_work`, `migrate_one_context`) that may
+    /// access a PercpuBlock whose `context::init()` has not run yet
+    /// during AP bring-up.
+    ///
+    /// # Returns
+    /// `Some(Arc<ContextLock>)` if the idle context is set, `None` otherwise.
+    pub fn try_idle_context(&self) -> Option<Arc<ContextLock>> {
+        self.idle_ctxt.borrow().as_ref().map(Arc::clone)
+    }
 }