feat(bootstrap): function pointer types + C emission

- Add tekFunc AST node and parser support for func(Params) -> Ret
- Add tkFunc type resolution in sema and hir_lower
- Add &FuncName address-taking yielding tkMutRef(tkFunc)
- Fix C emission: function pointer params use Ret (*name)(Params) syntax
  via cParamDecl helper that embeds the name inside (*)
- Fix forward declarations and temp declarations for function pointers
- Add funcPtrTypes table in C backend so &Fn temps get proper type
- Add _test_funcptr integration test

bootstrap/ast.nim

@@ -33,6 +33,7 @@ type
     tekDynRef    ## &dyn Trait — trait object (fat pointer)
     tekTuple
     tekSelf
+    tekFunc
 
   TypeExpr* = ref object
     loc*: SourceLocation
@@ -54,6 +55,9 @@ type
       tupleElements*: seq[TypeExpr]
     of tekSelf:
       discard
+    of tekFunc:
+      funcParams*: seq[TypeExpr]
+      funcRet*: TypeExpr
 
   # ---------------------------------------------------------------------------
   # Patterns