wake application processors to long mode

INIT-SIPI-SIPI plus a self-relocating real-mode trampoline.

build.zig

@@ -73,6 +73,9 @@ pub fn build(b: *std.Build) void {
     // CPU-exception stubs — real assembly, since they need cross-symbol
     // jumps/calls that Zig inline asm can't express (see the file's header).
     arch_mod.addAssemblyFile(b.path("src/kernel/arch/x86_64/isr.s"));
+    // The AP bring-up trampoline: 16-/32-/64-bit mode-switch code that can't be
+    // inline asm (it runs relocated to a low page, not at its link address).
+    arch_mod.addAssemblyFile(b.path("src/kernel/arch/x86_64/trampoline.s"));
 
     // Firmware-agnostic device discovery. The generic kernel imports this as
     // "platform" and asks it to enumerate hardware into a backend-neutral device