Zero-initialize BlockIdExt::{root,file}_hash in default constructor

[DanShaders]

Let's not create unnecessary footguns.

[github-actions]

• tdactor/td/actor/coro_task.h:142-175 – promise_type no longer overrides return_value(T), so co_return {} now matches the empty ExternalResult overload and leaves Result<T> default-constructed (error -1). Tasks that do co_return {} (common with Unit) will again report failures instead of success; this regresses the fix added in 2384d66.
• .gitmodules, CMake/BuildLibbacktrace.cmake, CMakeLists.txt:4-14/194-276, tdutils/td/utils/port/stacktrace.cpp – the diff removes the libbacktrace submodule, its build integration, and the enhanced stacktrace path (and dSYM generation) while the PR title only mentions BlockIdExt initialization. This is a major behavior change that drops the improved stack traces; please confirm if this wide-scope revert is intentional.

Open question: Was the libbacktrace/stacktrace removal intended, or is this branch based on an older snapshot?