fix(stress): extend optional chain through .trim() in readHostCounters

?.[1] short-circuits to undefined when /proc/stat's cpu line is missing,
but the following .trim() ran unconditionally and threw before the ?? []
fallback could apply — breaking readHostCounters' "return null for any
source the platform lacks" contract and surfacing as an uncaught
exception in the setInterval sampling callback.

Addresses PR #617 review threads from gemini-code-assist and claude.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: kriszyp

integrationTests/stress/stressShared.mjs

@@ -63,7 +63,7 @@ export function readHostCounters() {
 	const cpu =
 		stat
 			.match(/^cpu\s+(.*)$/m)?.[1]
-			.trim()
+			?.trim()
 			.split(/\s+/)
 			.map(Number) ?? [];
 	const [, , , idle = 0, iowait = 0, , , steal = 0] = cpu;