cgen, coroutines: fix go expr handle referencing undeclared thread__tN

[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_<tmp> 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_<tmp>. The generated C now contains:

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.

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