medvednikov
## Description
Fixes #27338.
With `-use-coroutines`, a `go expr` whose handle is stored/used (e.g. `a.worker = go a.loop()`) made cgen reference a C variable `thread__tN` that it never declared:
```
error: use of undeclared identifier 'thread__t2'
a.worker = /*go (coroutine) */ thread__t2;
```
The coroutine branch in `spawn_and_go_expr` only emitted the `photon_thread_create*` call (which returns `void` and yields no joinable handle), yet the handle `thread_` was still written back as the expression's value.
## Fix
The Photon wrapper exposes no waitable handle, so a `go` whose handle is **actually used** (`node.is_expr`) is now lowered to the regular `spawn` (pthread) path, which declares and assigns `thread_`. The generated C now contains:
```c
pthread_t thread__t2;
...
int _t2_thr_res = pthread_create(&thread__t2, ..., main__App_loop_thread_wrapper, arg__t2);
a.worker = thread__t2;
```
so it compiles **and** the handle is joinable via `.wait()`, matching the non-coroutine `spawn` path. Plain statement-form `go f()` (no handle used) is unchanged and keeps using the Photon work-pool path.
A companion change in `markused` is required: a `go` handle that is used must mark the same thread-handle type and pthread error helpers (`panic_error_number`, `c_error_number_str`, …) that `spawn` needs — otherwise `-skip-unused` (the default) prunes them and the rerouted spawn path fails to link with `undefined symbol 'builtin__panic_error_number'`.
## Verification
- The reproducer now generates a properly-declared `pthread_t` handle (was: undeclared identifier).
- `panic_error_number` / `c_error_number_str` are emitted (no longer pruned).
- The compiler self-compiles cleanly, and existing spawn/thread tests pass (`array_of_threads_wait_test.v`, `spawn_with_cond_fncall_test.v`, `generic_spawn_test.v`).
- Added `examples/coroutines/go_handle.v` documenting the now-working pattern.
> Note: an automated CI test isn't included because `-use-coroutines` requires the Photon `.so` (downloaded at build time, not available on CI) — consistent with the existing coroutines examples being marked `vtest build: false`.
medvednikov