feat(boot+shell): add bootable disk image, unified scheduler and interactive user-space shell

- Implement Limine v8 boot protocol requests in dedicated PT_LOAD RW segment
- Configure IOPL=3 and SSE in arch setup for Ring 3 drivers and libc
- Unify scheduler context switching via switch_to and preempt_tick
- Support dual-source keyboard/serial input in sys_key_read
- Add automated 'make image' and 'make run' targets for instant QEMU launch
- Complete interactive UNIX-like shell (osh) with built-in commands & ramdisk

kernel/asm/context.S

@@ -1,14 +1,15 @@
-/* opencoreC - Thread Context Switch */
+/* opencoreC - Thread Context Switch & Entry Trampoline */
 .global switch_to
+.global thread_entry_trampoline
 
 .section .text
 /*
- * void switch_to(uint64_t **prev_rsp_ptr, uint64_t *next_rsp);
- * RDI = pointer to current thread's stored RSP
- * RSI = target thread's RSP
+ * switch_to(uint64_t *prev_rsp_ptr, uint64_t next_rsp)
+ * RDI = &current_thread.context.rsp  (save current RSP here)
+ * RSI = next thread's saved RSP
  */
 switch_to:
-    /* Push callee-saved registers of outgoing thread */
+    /* Save callee-saved registers of outgoing thread */
     push rbp
     push rbx
     push r12
@@ -16,13 +17,13 @@ switch_to:
     push r14
     push r15
 
-    /* Save current stack pointer */
+    /* Save RSP of outgoing thread */
     mov [rdi], rsp
 
-    /* Load target stack pointer */
+    /* Load RSP of incoming thread */
     mov rsp, rsi
 
-    /* Pop callee-saved registers of incoming thread */
+    /* Restore callee-saved registers of incoming thread */
     pop r15
     pop r14
     pop r13
@@ -31,3 +32,45 @@ switch_to:
     pop rbp
 
     ret
+
+/*
+ * thread_entry_trampoline:
+ * Jumped to via 'ret' in switch_to when a newly created thread runs for the first time.
+ * When create_user_thread created the stack:
+ *   R12 was populated with user_rsp
+ *   R13 was populated with entry_rip
+ */
+thread_entry_trampoline:
+    /* Set user data segments (0x18 | 3 = 0x1B) */
+    mov ax, 0x1B
+    mov ds, ax
+    mov es, ax
+    mov fs, ax
+    mov gs, ax
+
+    /* Build hardware IRETQ frame on kernel stack */
+    push 0x1B          /* User SS */
+    push r12           /* User RSP */
+    push 0x3202        /* RFLAGS (IF=1, IOPL=3 for hardware drivers) */
+    push 0x23          /* User CS (0x20 | 3 = 0x23) */
+    push r13           /* User RIP */
+
+    /* Clear all general-purpose registers */
+    xor rax, rax
+    xor rbx, rbx
+    xor rcx, rcx
+    xor rdx, rdx
+    xor rsi, rsi
+    xor rdi, rdi
+    xor rbp, rbp
+    xor r8,  r8
+    xor r9,  r9
+    xor r10, r10
+    xor r11, r11
+    xor r12, r12
+    xor r13, r13
+    xor r14, r14
+    xor r15, r15
+
+    /* Transition to Ring 3 */
+    iretq