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

library/std/Os.bux

@@ -0,0 +1,34 @@
+module Std::Os {
+
+extern func bux_argc() -> int;
+extern func bux_argv(index: int) -> String;
+extern func bux_getenv(name: String) -> String;
+extern func bux_setenv(name: String, value: String) -> int;
+extern func bux_getcwd() -> String;
+extern func bux_chdir(path: String) -> int;
+
+func Os_ArgsCount() -> int {
+    return bux_argc();
+}
+
+func Os_Args(index: int) -> String {
+    return bux_argv(index);
+}
+
+func Os_GetEnv(name: String) -> String {
+    return bux_getenv(name);
+}
+
+func Os_SetEnv(name: String, value: String) -> bool {
+    return bux_setenv(name, value) == 0;
+}
+
+func Os_GetCwd() -> String {
+    return bux_getcwd();
+}
+
+func Os_Chdir(path: String) -> bool {
+    return bux_chdir(path) == 0;
+}
+
+}