Phase 2: Fix sema stub — add full function body type checking

- Iterate function body statements and call Sema_CheckStmt on each
- Add Sema_CheckBlock helper for child scope creation in if/while/for/loop blocks
- Extend Sema_CheckStmt to handle all statement kinds: skIf, skWhile, skDoWhile,
  skLoop, skFor, skMatch, skBreak, skContinue, skDecl, skReturn
- Extend Sema_CheckExpr with ekAssign, ekField, ekIndex, ekBlock, ekSelf
- Fix ekCall to resolve return type from function declaration
- Fix ekBinary to handle assignment operators (tkAssign, +=, -=, etc.)
- Add Sema_CollectGlobals handling for dkUse (imports), dkConst, dkEnum variants
- Add tkColonColon path parsing in parser (Color::Red)
- Add decl field to Symbol struct for function/type resolution
- Add Sema_ZeroInitSymbol helper to avoid garbage from uninitialized struct fields
- Fix concurrency.bux missing Task_Join import

All examples now pass buxc2 check. make selfhost succeeds.

src_bux/cli.bux

@@ -170,6 +170,7 @@ func Cli_Check(srcPath: String) -> int {
         PrintLine("Parse failed");
         return 1;
     }
+    PrintLine("  Parse done");
 
     // Flatten module wrappers
     var decl2: *Decl = mod.firstItem;
@@ -185,6 +186,14 @@ func Cli_Check(srcPath: String) -> int {
     let sema: *Sema = Sema_Analyze(mod);
     if Sema_HasError(sema) {
         PrintLine("Sema errors found");
+        var i: int = 0;
+        while i < Sema_DiagCount(sema) {
+            Print("  line ");
+            PrintInt(sema.diags[i].line as int64);
+            Print(": ");
+            PrintLine(sema.diags[i].message);
+            i = i + 1;
+        }
         return 1;
     }