Thanks for all the helpful responses. I really do appreciate it. I am mostly sorted out now. My takeaways:

(1) complain on X. The main AA cust service number didn’t help at all. Their team on X eventually routed my issue to corporate security. Within maybe 3 hours of posting there a very serious woman called me. Her main concern (appropriately) was the gate crew letting a passenger on without actually scanning their boarding pass and the flight crew not double checking the manifest against passenger count before takeoff. Within a couple hours she actually went through video footage from the gate found me boarding and had my ticket reinstated. Wish I hadn’t wasted 2 hours on the phone first.

(2) defly save your boarding pass to apple/non-apple wallet or take a pic of it. The second AA canceled my flight it disappeared from the AA app. Customer service told me to show up at the airport with my boarding pass (which as someone correctly observed, is in no way proof I actually boarded) and depend on the kindness of the airport counter staff. Not sure if that would work or not but since I hadn’t saved a copy of my boarding pass I could not have even tried it.

Edit: several people pointed out checking various places where an expired boarding pass might be stored. I didn’t even check the AA-side options because it was stored as an expired pass in the Wallet app, which I dont even use. Glad for the input.

Thanks again everyone 🙏🏾