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/cli.bux

@@ -31,7 +31,7 @@ func DirExists(path: String) -> bool {
 }
 
 // Import the compiler pipeline
-// In self-hosting mode, these are compiled together from src_bux/
+// In self-hosting mode, these are compiled together from compiler/selfhost/
 
 // ---------------------------------------------------------------------------
 // Compile a single .bux source file
@@ -140,19 +140,19 @@ func Cli_Build(srcPath: String, outPath: String) -> int {
     PrintLine(cFile);
 
     // Find runtime.c and io.c for linking
-    var rtPath: String = "stdlib/runtime.c";
-    var ioPath: String = "stdlib/io.c";
+    var rtPath: String = "library/runtime/runtime.c";
+    var ioPath: String = "library/runtime/io.c";
     if !FileExists(rtPath) {
-        rtPath = "../stdlib/runtime.c";
+        rtPath = "../library/runtime/runtime.c";
     }
     if !FileExists(ioPath) {
-        ioPath = "../stdlib/io.c";
+        ioPath = "../library/runtime/io.c";
     }
     if !FileExists(rtPath) {
-        rtPath = "/home/ziko/z-git/bux/bux/stdlib/runtime.c";
+        rtPath = "/home/ziko/z-git/bux/bux/library/runtime/runtime.c";
     }
     if !FileExists(ioPath) {
-        ioPath = "/home/ziko/z-git/bux/bux/stdlib/io.c";
+        ioPath = "/home/ziko/z-git/bux/bux/library/runtime/io.c";
     }
 
     // Compile with cc
@@ -633,16 +633,16 @@ func Cli_BuildProject(projectDir: String) -> int {
     var rtPath: String = bux_path_join(stdlibDir, "runtime.c");
     var ioPath: String = bux_path_join(stdlibDir, "io.c");
     if !FileExists(rtPath) {
-        rtPath = bux_path_join(projectDir, "stdlib/runtime.c");
+        rtPath = bux_path_join(projectDir, "library/runtime/runtime.c");
     }
     if !FileExists(ioPath) {
-        ioPath = bux_path_join(projectDir, "stdlib/io.c");
+        ioPath = bux_path_join(projectDir, "library/runtime/io.c");
     }
     if !FileExists(rtPath) {
-        rtPath = bux_path_join(projectDir, "../stdlib/runtime.c");
+        rtPath = bux_path_join(projectDir, "../library/runtime/runtime.c");
     }
     if !FileExists(ioPath) {
-        ioPath = bux_path_join(projectDir, "../stdlib/io.c");
+        ioPath = bux_path_join(projectDir, "../library/runtime/io.c");
     }
 
     // Compile with cc

_selfhost/src/scope.bux

@@ -9,7 +9,7 @@ const skConst: int = 3;
 const skModule: int = 4;
 
 // Maximum symbols per scope
-const maxSymbols: int = 1024;
+const maxSymbols: int = 8192;
 
 struct Symbol {
     kind: int;

_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 {