A hung page (a runaway Runtime.evaluate, a synchronous DOM op) holds the
process-wide V8 lock and wedges every other session forever. The one-shot CLI has
a process-level hard deadline for this; the long-running server cannot force-exit,
so terminate just the offending isolate instead.
dispatch() arms a watchdog on the session page's isolate after taking the V8 lock
and disarms it after the handler; on overrun it terminates the isolate and clears
the termination flag so the session keeps working. OBSCURA_CDP_COMMAND_TIMEOUT_MS
tunes the bound (default 60s, 0 disables); navigation already self-bounds well
under it.
The watchdog is a single long-lived shared thread (cdp_watchdog), not a thread
per command: the V8 lock serializes commands so at most one is ever armed, and
arm/disarm are a mutex plus notify with no measurable per-command cost. Also
fixes a lost-wakeup in the per-call watchdog (spawn_watchdog) where disarming a
fast call before the watchdog thread started blocked for the full budget.
Verified: a hung Runtime.evaluate is terminated at the budget and the session
recovers; CDP integration 37/37, obstacle course 33/33; per-command latency
unchanged (watchdog on equals off within noise).
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