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/selfhost/scope.bux

@@ -0,0 +1,95 @@
+// scope.bux — Symbol table with parent-chain lookup
+module Scope {
+
+// Symbol kinds
+const skVar: int = 0;
+const skFunc: int = 1;
+const skType: int = 2;
+const skConst: int = 3;
+const skModule: int = 4;
+
+// Maximum symbols per scope
+const maxSymbols: int = 8192;
+
+struct Symbol {
+    kind: int;
+    name: String;
+    typeKind: int;
+    typeName: String;
+    isMutable: bool;
+    isPublic: bool;
+    decl: *Decl;            // associated declaration (for funcs, structs, enums)
+}
+
+struct Scope {
+    symbols: *Symbol;
+    count: int;
+    parent: *Scope;
+}
+
+// ---------------------------------------------------------------------------
+// Scope operations
+// ---------------------------------------------------------------------------
+
+func Scope_New() -> Scope {
+    let sz: uint = maxSymbols as uint * sizeof(Symbol);
+    let data: *Symbol = bux_alloc(sz) as *Symbol;
+    return Scope { symbols: data, count: 0, parent: null as *Scope };
+}
+
+func Scope_NewChild(parent: *Scope) -> Scope {
+    let sz: uint = maxSymbols as uint * sizeof(Symbol);
+    let data: *Symbol = bux_alloc(sz) as *Symbol;
+    return Scope { symbols: data, count: 0, parent: parent };
+}
+
+func Scope_Define(scope: *Scope, sym: Symbol) -> bool {
+    // Check local scope for duplicates
+    var i: int = 0;
+    while i < scope.count {
+        if String_Eq(scope.symbols[i].name, sym.name) {
+            return false;
+        }
+        i = i + 1;
+    }
+    if scope.count < maxSymbols {
+        scope.symbols[scope.count] = sym;
+        scope.count = scope.count + 1;
+        return true;
+    }
+    return false;
+}
+
+func Scope_Lookup(scope: *Scope, name: String) -> Symbol {
+    var cur: *Scope = scope;
+    while cur != null as *Scope {
+        var i: int = 0;
+        while i < cur.count {
+            if String_Eq(cur.symbols[i].name, name) {
+                return cur.symbols[i];
+            }
+            i = i + 1;
+        }
+        cur = cur.parent;
+    }
+    var empty: Symbol = Symbol { kind: 0, name: "", typeKind: 0, typeName: "", isMutable: false, isPublic: false, decl: null as *Decl };
+    return empty;
+}
+
+func Scope_LookupLocal(scope: *Scope, name: String) -> Symbol {
+    var i: int = 0;
+    while i < scope.count {
+        if String_Eq(scope.symbols[i].name, name) {
+            return scope.symbols[i];
+        }
+        i = i + 1;
+    }
+    var empty: Symbol = Symbol { kind: 0, name: "", typeKind: 0, typeName: "", isMutable: false, isPublic: false, decl: null as *Decl };
+    return empty;
+}
+
+func Scope_Free(scope: *Scope) {
+    bux_free(scope.symbols as *void);
+}
+
+}