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/parser.bux

@@ -413,6 +413,19 @@ func parserParsePostfix(p: *Parser) -> *Expr {
             continue;
         }
 
+        // Path: expr::name
+        if kind == tkColonColon {
+            discard parserAdvance(p);
+            let line: uint32 = parserCurToken(p).line;
+            let col: uint32 = parserCurToken(p).column;
+            let name: LexToken = parserExpect(p, tkIdent, "expected path segment");
+            let e: *Expr = parserMakeExpr(ekField, line, col);
+            e.child1 = left;
+            e.strValue = name.text;
+            left = e;
+            continue;
+        }
+
         // as, is
         if kind == tkAs || kind == tkIs {
             discard parserAdvance(p);