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/scope.bux

@@ -18,6 +18,7 @@ struct Symbol {
     typeName: String;
     isMutable: bool;
     isPublic: bool;
+    decl: *Decl;            // associated declaration (for funcs, structs, enums)
 }
 
 struct Scope {
@@ -90,4 +91,5 @@ func Scope_LookupLocal(scope: *Scope, name: String) -> Symbol {
 func Scope_Free(scope: *Scope) {
     bux_free(scope.symbols as *void);
 }
+
 }