feat(kernel): add preemptive round-robin thread scheduler

include/sched.h

@@ -0,0 +1,45 @@
+#ifndef FROSTBYTE_SCHED_H
+#define FROSTBYTE_SCHED_H
+
+#include "types.h"
+#include "vfs.h"
+
+typedef enum {
+    threadUnused = 0,
+    threadRunnable = 1,
+    threadRunning = 2,
+    threadSleeping = 3,
+    threadDead = 4
+} ThreadState;
+
+typedef void (*ThreadEntry)(void);
+
+typedef struct Thread {
+    uint64 rsp;
+    uint64 stackBase;
+    uint64 kernelStackTop;
+    uint64 sleepUntil;
+    uint64 brk;
+    uint32 id;
+    ThreadState state;
+    const char* name;
+    FileDescriptor fds[maxOpenFiles];
+} Thread;
+
+void schedInit(void);
+uint32 threadCreate(const char* name, ThreadEntry entry);
+uint32 threadCreateUser(const char* name, uint64 entry, uint64 userStackTop);
+uint64 schedule(uint64 currentRsp);
+void schedStart(void);
+
+void threadExit(uint64 code);
+void threadSleep(uint64 milliseconds);
+void threadSetBrk(uint32 id, uint64 brk);
+uint64 currentThreadSbrk(int64 increment);
+
+uint32 threadCount(void);
+Thread* threadAt(uint32 index);
+Thread* currentThread(void);
+uint32 currentThreadId(void);
+
+#endif