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/Time.bux

@@ -0,0 +1,19 @@
+module Std::Time {
+
+extern func bux_time_ms() -> int64;
+extern func bux_time_us() -> int64;
+extern func bux_sleep_ms(ms: int64);
+
+func Time_NowMs() -> int64 {
+    return bux_time_ms();
+}
+
+func Time_NowUs() -> int64 {
+    return bux_time_us();
+}
+
+func Time_SleepMs(ms: int64) {
+    bux_sleep_ms(ms);
+}
+
+}