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/Math.bux

@@ -0,0 +1,44 @@
+module Std::Math {
+
+extern func bux_sqrt(x: float64) -> float64;
+extern func bux_pow(x: float64, y: float64) -> float64;
+extern func bux_abs_i64(x: int64) -> int64;
+extern func bux_abs_f64(x: float64) -> float64;
+extern func bux_min_i64(a: int64, b: int64) -> int64;
+extern func bux_max_i64(a: int64, b: int64) -> int64;
+extern func bux_min_f64(a: float64, b: float64) -> float64;
+extern func bux_max_f64(a: float64, b: float64) -> float64;
+
+func Sqrt(x: float64) -> float64 {
+    return bux_sqrt(x);
+}
+
+func Pow(x: float64, y: float64) -> float64 {
+    return bux_pow(x, y);
+}
+
+func Abs(n: int64) -> int64 {
+    return bux_abs_i64(n);
+}
+
+func AbsF(f: float64) -> float64 {
+    return bux_abs_f64(f);
+}
+
+func Min(a: int64, b: int64) -> int64 {
+    return bux_min_i64(a, b);
+}
+
+func Max(a: int64, b: int64) -> int64 {
+    return bux_max_i64(a, b);
+}
+
+func MinF(a: float64, b: float64) -> float64 {
+    return bux_min_f64(a, b);
+}
+
+func MaxF(a: float64, b: float64) -> float64 {
+    return bux_max_f64(a, b);
+}
+
+}