Embedding SELF_HEAL_INTERVAL_MS latches

Chose permanently latching only localStackMissing over permanently disabling every failed embedding worker because recovery from a missing optional stack requires reinstall, while WASM-fatal errors, floor OOM, transient-init exhaustion, and non-zero exits may recover with a fresh worker. Probe those recoverable states every SELF_HEAL_INTERVAL_MS (6 hours). lastSelfHealAt = 0 means no probe has run, so the first tick is primed.