perf(cpu): tune streaming playback

Keep CPU async decode enabled without CFG parallelism, expand CPU buffering defaults for smooth playback, prevent CPU startup from mutating the lockfile during thread autodetection, and document runtime tuning variables in the example environment file.

web/components/GenerationControls.tsx

@@ -157,7 +157,7 @@ export default function GenerationControls({
       <div className="flex flex-col gap-2">
         <div className="flex items-center justify-between">
           <label className="text-sm font-medium" style={{ color: "var(--foreground)" }}>
-            Quality vs Speed
+            Speed vs Quality
           </label>
           <span
             className="text-sm font-mono px-2 py-0.5 rounded"
@@ -221,7 +221,7 @@ export default function GenerationControls({
             <input
               type="range"
               min={0.5}
-              max={10.0}
+              max={30.0}
               step={0.5}
               value={prebufferSecs}
               onChange={(e) => onPrebufferSecsChange(parseFloat(e.target.value))}
@@ -271,7 +271,7 @@ export default function GenerationControls({
               id="resume-threshold"
               type="range"
               min={0.5}
-              max={5.0}
+              max={30.0}
               step={0.1}
               value={resumeThresholdSecs}
               onChange={(e) => {