For sudden cardiac death? Isn't that mostly caused by arrhythmia? So a prophylactic pacemaker maybe? Isn't that already a thing?

The threat model could be a freak heart stabbing. In which case a backup heart somewhere else (and some truly earthshattering breakthroughs in fast clotting) might make sense.

My dream technology would be a way to make my body just ... stop, if Something Bad happened, like all my blood falling out, until someone stumbled upon my non-rotting corpse, patched the holes, and filled it back up.

It's such bullshit that if I stop living for even a little bit my body melts into useless slag like an engine running without oil.

I imagined damage to the heart muscles or blocked blood vessel.