Oasis ships no test runner, so this is a dependency-free node script (node
test/karvan_model.test.js). Covers rooms, ephemeral messages, the 250-msg ring
buffer, truncation/validation, self-destruct by idle+absolute TTL, idle reset on
activity, MAX_ROOMS eviction, and the WebRTC signaling mailbox. Lives in test/
(outside src/), so it is not bundled into the APK.
