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

README.md

@@ -149,7 +149,7 @@ func Main() -> int {
 | **Methods** | `extend` blocks for struct methods |
 | **Interfaces** | `interface` + `extend` for trait-like behavior |
 | **Error Handling** | `Result<T,E>`, `Option<T>`, and the `?` operator |
-| **Standard Library** | `Io`, `Array`, `String`, `Map`, `Fs`, `Mem`, `Set`, `Path`, `Math`, `Task`, `Channel` |
+| **Standard Library** | `Io`, `Array`, `String`, `Map`, `Fs`, `Mem`, `Set`, `Path`, `Math`, `Task`, `Channel`, `Os`, `Time`, `Process` |
 | **Backend** | C transpiler (self-hosting + bootstrap) |
 | **Strings** | Raw multi-line backtick strings (`...`), C-string interop |
 | **Gradual Ownership** | `@[Checked]` + `&T`/`&mut T` borrow checking |
@@ -166,18 +166,22 @@ func Main() -> int {
 
 ```
 bux/
-├── src/              # Bootstrap compiler (Nim)
-├── src_bux/          # Self-hosting compiler source (Bux)
-├── _selfhost/        # Self-hosting build artifacts
-├── stdlib/           # Standard library (.bux + .c runtime)
-│   └── Std/
-│       ├── Io.bux, String.bux, Array.bux, Map.bux
-│       ├── Fs.bux, Mem.bux, Set.bux
-│       ├── Path.bux, Math.bux
-│       └── Task.bux, Channel.bux
+├── compiler/         # Compiler source code
+│   ├── bootstrap/    # Bootstrap compiler (Nim)
+│   ├── selfhost/     # Self-hosting compiler source (Bux)
+│   └── tests/        # Compiler unit tests (Nim)
+├── library/          # Standard library
+│   ├── std/          # Standard library modules (.bux)
+│   │   ├── Io.bux, String.bux, Array.bux, Map.bux
+│   │   ├── Fs.bux, Mem.bux, Set.bux
+│   │   ├── Path.bux, Math.bux
+│   │   └── Task.bux, Channel.bux
+│   └── runtime/      # C runtime files (runtime.c, io.c)
+├── tests/            # Language integration tests
 ├── examples/         # Example programs
-├── tests/            # Unit tests (Nim)
+├── tools/            # Additional tooling
 ├── docs/             # Documentation
+├── buxs/             # Windows-compatible project root
 ├── README.md
 ├── PLAN.md           # Roadmap to self-hosting
 └── Makefile
@@ -204,6 +208,8 @@ make test-examples
 make clean
 ```
 
+> **Windows users:** Use the `buxs/` directory as your project root to avoid path conflicts.
+
 ---
 
 ## Documentation