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_parse_ast.nim

@@ -0,0 +1,16 @@
+import "../bootstrap/lexer", "../bootstrap/parser", "../bootstrap/ast", "../bootstrap/token"
+import std/os
+
+let source = readFile("../_selfhost/src/ast.bux")
+let lexRes = tokenize(source, "ast.bux")
+echo "Tokens: ", lexRes.tokens.len
+echo "Errors: ", lexRes.hasErrors
+let res = parse(lexRes.tokens, "ast.bux")
+if res.module == nil:
+  echo "Parse failed, diagnostics: ", res.diagnostics.len
+  for d in res.diagnostics:
+    echo "  ", d.message
+else:
+  echo "Parsed OK, items: ", res.module.items.len
+  for i, decl in res.module.items:
+    echo "Decl ", i, " kind=", decl.kind, " name=", decl.declFuncName