feat: restructure repo, borrow checker, expanded stdlib

- Reorganize repository to Rust-style layout:
  compiler/bootstrap/  compiler/selfhost/  compiler/tests/
  library/std/  library/runtime/  tests/  tools/
- Add buxs/ Windows-compatible project root
- Add borrow checker tests and implement:
  - Alias analysis (double mutable borrow detection)
  - Use-after-move detection for own T
- Expand standard library:
  - Std::Os: Args, Env, Cwd, Chdir
  - Std::Time: NowMs, NowUs, SleepMs
  - Std::Process: Run, Output
  - Std::Io: PrintInt64 (fixes 32-bit truncation bug)
- Add examples: os_time.bux, process.bux
- Fix PrintInt to use int64_t in C runtime

library/std/Task.bux

@@ -0,0 +1,23 @@
+module Std::Task {
+
+extern func bux_task_spawn(fn: *void, arg: *void) -> *void;
+extern func bux_task_join(handle: *void);
+extern func bux_task_sleep(ms: int64);
+
+struct TaskHandle {
+    handle: *void;
+}
+
+func Task_Spawn(fn: *void, arg: *void) -> TaskHandle {
+    return TaskHandle { handle: bux_task_spawn(fn, arg) };
+}
+
+func Task_Join(t: TaskHandle) {
+    bux_task_join(t.handle);
+}
+
+func Task_Sleep(ms: int64) {
+    bux_task_sleep(ms);
+}
+
+}