cgen, coroutines: `go expr` handle references undeclared `thread__tN` C variable

[medvednikov]

Description

With -use-coroutines, a go expr whose handle is stored/used makes cgen reference a C variable thread__tN that it never declares, so the generated C fails to compile.

V version: V 0.5.1, current master (27f394507)
OS/Backend: reproduced on macOS arm64; originally reported on Linux/cc. Requires -use-coroutines.

Reproducer

struct App {
mut:
	worker thread
}

fn (a App) loop() {}

fn main() {
	mut a := App{}
	a.worker = go a.loop()
	a.worker.wait()
}

Compile with:

v -use-coroutines file.v

Current behavior

C compilation fails — V references an undeclared thread__tN:

error: use of undeclared identifier 'thread__t2'
 4112 | thread__t2;

The generated C for the coroutine handle is:

a.worker = /*go (coroutine) */ thread__t2;

The coroutine branch emits the photon_thread_create_and_migrate_to_work_pool(...) call but never declares the thread__tN handle variable it then reads.

Expected behavior

Compiles and runs (as the non-coroutine spawn path does).

Notes

• Reported via the bugs.vlang.io crash reporter (original symbol thread__t4, from db.flush_thread = go db.flush_loop()).
• Likely in vlib/v/gen/c/spawn_and_go.v: the is_go (coroutine) branch sets the handle expression to thread_${tmp} without emitting its declaration, unlike the regular spawn path.