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

_selfhost/src/sema.bux

@@ -438,6 +438,7 @@ func Sema_CheckStmt(sema: *Sema, stmt: *Stmt) {
 
 func Sema_CollectGlobals(sema: *Sema) {
     var decl: *Decl = sema.module.firstItem;
+    var funcCount: int = 0;
     while decl != null as *Decl {
         let dk: int = decl.kind;
 
@@ -503,6 +504,24 @@ func Sema_CollectGlobals(sema: *Sema) {
             }
         }
 
+        // Const
+        if dk == dkConst {
+            var sym: Symbol;
+            Sema_ZeroInitSymbol(&sym);
+            sym.kind = skConst;
+            sym.name = decl.strValue;
+            if decl.constType != null as *TypeExpr {
+                sym.typeKind = Sema_ResolveType(sema, decl.constType);
+                sym.typeName = decl.constType.typeName;
+            } else {
+                sym.typeKind = tyInt;
+            }
+            sym.isMutable = false;
+            sym.isPublic = decl.isPublic;
+            sym.decl = decl;
+            discard Scope_Define(sema.scope, sym);
+        }
+
         // Use (import)
         if dk == dkUse {
             if decl.useKind == 2 {