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

examples/process.bux

@@ -0,0 +1,18 @@
+// Process - Run external commands and capture output
+import Std::Io::{PrintLine, PrintInt};
+import Std::Process::{Process_Run, Process_Output};
+
+func Main() -> int {
+    PrintLine("Process demo:");
+    
+    let output: String = Process_Output("echo Hello from subprocess");
+    Print("Captured: ");
+    PrintLine(output);
+    
+    let rc: int = Process_Run("true");
+    Print("Exit code: ");
+    PrintInt(rc);
+    PrintLine("");
+    
+    return 0;
+}