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/Io.bux

@@ -0,0 +1,28 @@
+module Std::Io {
+
+extern func PrintLine(s: String);
+extern func Print(s: String);
+extern func PrintInt(n: int);
+extern func PrintInt64(n: int64);
+extern func PrintFloat(f: float64);
+extern func PrintBool(b: bool);
+extern func ReadLine() -> String;
+extern func bux_read_file(path: String) -> String;
+extern func bux_write_file(path: String, content: String) -> int;
+extern func bux_file_exists(path: String) -> int;
+
+func ReadFile(path: String) -> String {
+    return bux_read_file(path);
+}
+
+func WriteFile(path: String, content: String) -> bool {
+    let r: int = bux_write_file(path, content);
+    return r != 0;
+}
+
+func FileExists(path: String) -> bool {
+    let r: int = bux_file_exists(path);
+    return r != 0;
+}
+
+}