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

compiler/tests/test_ast_fields.nim

@@ -0,0 +1,15 @@
+import "../bootstrap/lexer", "../bootstrap/parser", "../bootstrap/ast"
+
+let source = readFile("../_selfhost/src/ast.bux")
+let lexRes = tokenize(source, "ast.bux")
+let res = parse(lexRes.tokens, "ast.bux")
+if res.module != nil:
+  echo "Module items: ", res.module.items.len
+  for decl in res.module.items:
+    if decl.kind == dkStruct:
+      echo "Struct: ", decl.declStructName, " fields: ", decl.declStructFields.len
+    elif decl.kind == dkModule:
+      echo "Inner module: ", decl.declModuleName, " items: ", decl.declModuleItems.len
+      for inner in decl.declModuleItems:
+        if inner.kind == dkStruct:
+          echo "  Struct: ", inner.declStructName, " fields: ", inner.declStructFields.len