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

buxs/README.md

@@ -0,0 +1,20 @@
+# BUXS — Windows-Compatible Project Root
+
+This directory serves as an alternative project root for Windows environments
+where the name `bux` may conflict with system paths or tooling.
+
+## Usage
+
+From this directory, you can build and run the Bux compiler using the
+provided wrapper scripts.
+
+## Structure
+
+The actual project source lives in the parent directory (`../`):
+
+- `../compiler/bootstrap/` — Nim bootstrap compiler
+- `../compiler/selfhost/` — Self-hosting compiler (Bux)
+- `../library/std/` — Standard library modules
+- `../library/runtime/` — C runtime files
+- `../tests/` — Integration tests
+- `../docs/` — Documentation