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/types.nim

@@ -183,6 +183,10 @@ proc isAssignableTo*(a, b: Type): bool =
       return true
     if b.isDynRef and a.isMutRef:
       return true
+  # &mut func(...) -> func(...) (function pointer decay)
+  if a.isMutRef and b.kind == tkFunc:
+    if a.inner.len > 0 and a.inner[0].kind == tkFunc:
+      return true
   return false
 
 # String representation