Never read sysfs through Go's poller, and a GPU that stops answering is skipped

On grid the sampler sat 42 minutes in runtime_pollWait on the Arc A380's
hwmon fan1_input: kernfs accepts epoll, so os.ReadFile made the fd
non-blocking, i915 answered EAGAIN once, and the poller waited for a
readiness kernfs only signals on sysfs_notify. Every graph on the machine
froze while the API carried on.

readStr now reads with plain blocking syscalls. Each GPU's sysfs reads run
under a three-second deadline; a device that does not answer is marked
stalled, logged, and left alone until restart, with the box saying so.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

web/src/Top.tsx

@@ -521,7 +521,8 @@ const ENGINES: [string, string][] = [['render', 'render'], ['compute', 'compute'
 function GPUBox({ g, values, tick, advance, cap }: { g: GPU; values: number[]; tick: number; advance: number; cap: number }) {
   const engines = ENGINES.filter(([k]) => g.engines && g.engines[k] !== undefined)
   const temp = g.temp !== undefined && g.temp > 0 ? g.temp : undefined
-  const how = g.usage_src === 'idle' ? 'busy, from idle residency' : g.usage_src === 'busy' ? 'busy, as the driver reports it' : g.usage_src === 'clients' ? 'the busiest engine, summed over its clients' : 'this driver exposes no activity counter'
+  const how = g.stalled ? 'a sysfs read of this card stopped answering; its readings are paused until the console restarts'
+    : g.usage_src === 'idle' ? 'busy, from idle residency' : g.usage_src === 'busy' ? 'busy, as the driver reports it' : g.usage_src === 'clients' ? 'the busiest engine, summed over its clients' : 'this driver exposes no activity counter'
   return (
     <div className="t-gpu">
       <div className="t-gpu-graph">