[feat] 100Hz task switch

kernel/scheduler/context_switch.S

@@ -5,7 +5,19 @@
 // rsi = new_rsp   (value of new stack pointer)
 //
 // Saves/restores callee-saved registers only.
-// On first switch into a new task, ret lands on task_entry_trampoline.
+// The caller-saved registers are already saved on the task's stack
+// by the ISR stub (for preemptive switching) or are not in use
+// (for the initial switch).
+//
+// For preemptive switching:
+//   - Timer IRQ fires, ISR stub pushes rax-r11 + trap_frame
+//   - C code runs (isr_dispatch → timer handler → scheduler)
+//   - context_switch saves/restores callee-saved regs + switches RSP
+//   - On return, C code unwinds, isr_common pops rax-r11 + iretq
+//
+// For new tasks:
+//   - task_create sets up a fake trap_frame on the stack
+//   - context_switch pops callee-saved regs, returns into task_entry_trampoline
 .global context_switch
 context_switch:
     push rbx