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

compiler/selfhost/source_location.bux

@@ -0,0 +1,13 @@
+// source_location.bux — Source position tracking
+module SourceLocation {
+
+struct SourceLocation {
+    line: uint32;
+    column: uint32;
+    offset: uint32;
+}
+
+func SourceLocation_New(line: uint32, column: uint32, offset: uint32) -> SourceLocation {
+    return SourceLocation { line: line, column: column, offset: offset };
+}
+}