The Wii U library links again

Once Cobalt's sign-in resolved a DID, the Wii U build failed at link time. The cache's TTLs were _Atomic time_t, and a 64-bit time_t has no lock-free atomic load on the 32-bit PowerPC. The linker wanted __atomic_load_8, which devkitPPC does not provide.

Fix

The TTLs are plain values under the cache's existing lock (#191, released in v0.38.2 via #192). The Wii U library now builds with no undefined references. Cobalt's .wuhb build is the real check, and it runs on each Cobalt PR.

Cobalt's sign-in branch that hit this is #207, which replaced the conflicting #202 and merged.