Stop sending a request again to a new translator child after the first one gave no answer for the read timeout.

plat_child_read returned -1 both for a broken pipe and for a child that sent nothing for PLAT_CHILD_READ_MS (120 s),
and svc_request retried any failure once on a fresh child. The translator is deterministic, so a program it hangs on
hangs the second child too: a compute shader, which the replay thread looks up synchronously (vk_exec_draw.c
shader_for), held the game for about 4 minutes instead of 2. plat_child_read now returns -2 on the timeout (win32.c and
linux.c), child_exchange_raw returns 1, 0 for a broken pipe or a child that could not start, and -1 for the timeout,
and svc_request stops the child and gives -1 without asking again; a dead child is still restarted once.
translate_one fails such a request with its own reason, keeps its job and code for diagnosis and deletes whatever the
stopped child wrote. The carry pass's child runs at idle priority, so a busy machine can starve it that long:
carry_exchange does not resend after a timeout either, and carry_translate counts it as a failed input, not as an
unavailable translator, so one slow program does not count towards the three that stop the pass. platform.h lets a
build set PLAT_CHILD_READ_MS.

Not reproduced before the change (no translator hang appears in the available logs). tests/child_timeout_check.py
(new) builds the real child and request functions with a 300 ms timeout: a silent child gives -2 and costs one
timeout with one child started, an exiting child is restarted once, an answering child gives its line. Every runtime
file compiles, with no new warnings in gfx_shader.c and win32.c.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/building.md

@@ -653,6 +653,12 @@ A change to which translations of a previous translator are served (`runtime/gfx
 `gfx_shader.c` and runs them against stand-in caches: a translation whose texture pool array has the earlier length
 (32768) is freed and the lookup goes on, one with the current length or no pool is served. No game files, no GPU.
 
+A change to how the translator children are read and restarted (`runtime/platform/win32.c` `plat_child_read`,
+`runtime/gfx_shader.c` `child_exchange_raw`, `svc_request`) passes `python tests\child_timeout_check.py`, which builds
+those functions with a 300 ms read timeout and runs `cmd.exe` children through them: one that never answers costs one
+timeout and is not asked again, one that exits is restarted once, one that answers gives its line. Windows only; no
+game files, no GPU.
+
 A change to where the lazy write-back is on by default (`runtime/wb_lazy_choice.h`, `runtime/vk_memory.c`
 `vk_mem_lazy_decide`) passes `python tests\wb_lazy_check.py`, which builds the header alone and checks the choice and
 its reason by platform, by GPU driver and by what the player set. No game files, no GPU.