schedule tasks across all cores

Per-core GDT/TSS and AP scheduler entry; fix AP SSE + single_threaded.

build.zig

@@ -114,7 +114,7 @@ pub fn build(b: *std.Build) void {
             .optimize = optimize,
             .code_model = .small, // kernel is linked in the low 2 GiB (see image_base)
             .red_zone = false, // interrupts would corrupt the SysV red zone
-            .single_threaded = true, // no scheduler yet; avoids pulling in TLS/atomics
+            .single_threaded = false, // SMP: the big kernel lock's atomics must be real across cores
             .sanitize_c = .off, // the UBSan runtime needs f128/SSE support we don't provide
             .stack_check = false, // stack-probe calls have no runtime to land in
             .stack_protector = false,