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

docs/BuildAndTest.md

@@ -34,7 +34,7 @@ make dev
 
 The output is a single binary: `buxc` (bootstrap compiler in Nim).
 
-The self-hosted compiler `buxc2` is built from `src_bux/*.bux` sources via:
+The self-hosted compiler `buxc2` is built from `compiler/selfhost/*.bux` sources via:
 ```bash
 make selfhost
 ```
@@ -140,7 +140,7 @@ bux/
 │   ├── hir_lower.nim # AST → HIR lowering
 │   ├── c_backend.nim # HIR → C code generation
 │   └── ...
-├── stdlib/           # Standard library
+├── library/           # Standard library
 │   ├── Std/
 │   │   ├── Io.bux
 │   │   ├── Array.bux
@@ -186,7 +186,7 @@ cat build/main.c
 
 | Error | Cause | Fix |
 |-------|-------|-----|
-| `stdlib directory not found` | `buxc` can't find `stdlib/` | Run from project root or set correct path |
+| `stdlib directory not found` | `buxc` can't find `library/` | Run from project root or set correct path |
 | `duplicate symbol 'bux_alloc'` | Multiple stdlib modules declare same extern | Only declare in one module |
 | `C compilation failed` | Generated C has errors | Check `build/main.c` for issues |
 

docs/Stdlib.md

@@ -454,8 +454,59 @@ func Main() -> int {
 - `Std::Mem` — Memory wrappers ✅
 - `Std::Set` — Hash set ✅
 - `Std::Path` — Path manipulation ✅
-- `Std::Os` — `Args`, `Env`, `Exit`, `Cwd` ⏳
-- `Std::Process` — Spawn subprocess ⏳
+- `Std::Os` — `Args`, `Env`, `Cwd`, `Chdir` ✅
+- `Std::Time` — `NowMs`, `NowUs`, `SleepMs` ✅
+- `Std::Process` — `Run`, `Output` ✅
 - `Std::Fmt` — String formatting with interpolation ⏳
 - `Std::Iter` — Iterator trait and combinators ⏳
-- `Std::Task` / `Std::Channel` — Lightweight concurrency (pthread-based threads) ✅
+- `Std::Task` / `Std::Channel` — Lightweight concurrency (pthread-based threads) ✅
+---
+
+## Std::Os
+
+Operating system interface.
+
+```bux
+import Std::Os::{Os_ArgsCount, Os_Args, Os_GetEnv, Os_SetEnv, Os_GetCwd, Os_Chdir};
+```
+
+| Function | Signature | Description |
+|----------|-----------|-------------|
+| `Os_ArgsCount` | `func Os_ArgsCount() -> int` | Number of command-line arguments |
+| `Os_Args` | `func Os_Args(index: int) -> String` | Get argument at index |
+| `Os_GetEnv` | `func Os_GetEnv(name: String) -> String` | Get environment variable |
+| `Os_SetEnv` | `func Os_SetEnv(name: String, value: String) -> bool` | Set environment variable |
+| `Os_GetCwd` | `func Os_GetCwd() -> String` | Get current working directory |
+| `Os_Chdir` | `func Os_Chdir(path: String) -> bool` | Change directory |
+
+---
+
+## Std::Time
+
+Time utilities.
+
+```bux
+import Std::Time::{Time_NowMs, Time_NowUs, Time_SleepMs};
+```
+
+| Function | Signature | Description |
+|----------|-----------|-------------|
+| `Time_NowMs` | `func Time_NowMs() -> int64` | Current time in milliseconds |
+| `Time_NowUs` | `func Time_NowUs() -> int64` | Current time in microseconds |
+| `Time_SleepMs` | `func Time_SleepMs(ms: int64)` | Sleep for N milliseconds |
+
+---
+
+## Std::Process
+
+Process spawning.
+
+```bux
+import Std::Process::{Process_Run, Process_Output};
+```
+
+| Function | Signature | Description |
+|----------|-----------|-------------|
+| `Process_Run` | `func Process_Run(cmd: String) -> int` | Run command, return exit code |
+| `Process_Output` | `func Process_Output(cmd: String) -> String` | Run command, capture stdout |
+