v0.31.2 — a dead tab is not a dead socket

A dead tab is not a dead socket, and restarting cannot fix one

wb go https://x.com/home timed out at 90s. The engine said the browser socket was half-dead,
reconnected (no effect), then advised restarting the engine, then restarting Chrome. None of
those remove a dead tab, which is what this actually was.

Measured: that x.com tab had been open for hours. Its websocket connected in 27 ms and
Runtime.evaluate 1 never returned — while /json/version and every other tab answered in
single-digit milliseconds. Chrome was healthy. The browser socket was healthy. One renderer was
gone. Closing that tab made x.com open instantly.

So on a read timeout the engine now asks this tab before blaming the connection:

"readError": "read: this tab's renderer had stopped responding — Chrome and the engine are
 both fine (Chrome answered raw CDP instantly; so did the other tabs). Closed the dead tab.
 Run the command again and it will open a fresh one.
 🔴 Do NOT restart the engine or Chrome for this — neither one would have removed the dead tab."

The probe is a fresh websocket to that one target plus a trivial evaluate, bounded at 2.5s, and
it only runs on a path that has already spent 30s. Unmeasurable returns null, not false — a tab
we could not measure is never closed.

Three agents spent hours on this, and one filed a request to switch the engine to raw CDP, which
would not have helped either: raw CDP failed on that tab exactly the same way. The automatic
dead-tab cleanup shipped in 0.13.9 lived only in rawcdp.js, so it ran only when the engine was
already on the raw-CDP fallback — never on the healthy playwright path where this happened.