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/bootstrap/scope.nim

@@ -0,0 +1,45 @@
+import std/tables
+import types, ast, source_location
+
+type
+  SymbolKind* = enum
+    skVar
+    skFunc
+    skType
+    skConst
+    skModule
+
+  Symbol* = ref object
+    kind*: SymbolKind
+    name*: string
+    typ*: Type
+    decl*: Decl        ## optional back-reference to AST decl
+    isMutable*: bool
+    isPublic*: bool
+
+  Scope* = ref object
+    parent*: Scope
+    table*: Table[string, Symbol]  ## O(1) lookup via hash table
+
+proc newScope*(parent: Scope = nil): Scope =
+  result = Scope(parent: parent)
+
+proc define*(scope: Scope, sym: Symbol): bool =
+  ## Returns false if name already exists in this scope
+  if scope.table.hasKey(sym.name):
+    return false
+  scope.table[sym.name] = sym
+  return true
+
+proc lookup*(scope: Scope, name: string): Symbol =
+  var cur = scope
+  while cur != nil:
+    if cur.table.hasKey(name):
+      return cur.table[name]
+    cur = cur.parent
+  return nil
+
+proc lookupLocal*(scope: Scope, name: string): Symbol =
+  if scope.table.hasKey(name):
+    return scope.table[name]
+  return nil