feat: introduce HOT! executable format and Ring 3 process isolation (v0.5.2)

- Implement custom 'HOT!' binary format and Rust-based elf2hot converter.
- Upgrade kernel loader with segment-based loading and BSS zeroing.
- Refactor scheduler for Ring 3 IRET frames and fix CS/SS selector swap.
- Add user stack allocation (0x70000000) and linker scripts for binary cleanup.

common/hot_header.h

@@ -0,0 +1,23 @@
+typedef unsigned char      u8;
+typedef unsigned short     u16;
+typedef unsigned int       u32;
+typedef unsigned long long u64; 
+
+#define HOT_MAGIC 0x21544F48
+
+typedef struct hot_segment {
+    u64 type; // 1 = rx 2 = rw
+    u64 vaddr; 
+    u64 offset;
+    u64 filesz;
+    u64 memsz;
+} hot_segment;
+
+typedef struct hot_header {
+    u32 magic; // "HOT!"
+    u8 version; // 1
+    u8 reserved_pad[3];
+    u64 entry_point;
+    u64 segments_count;
+    u64 reserved;
+} hot_header;