fix(identity): guard the DID cache TTLs with its lock, not atomics

[ewanc26]

What this changes

The DID document cache read its two TTLs through _Atomic time_t. On 32-bit PowerPC (the Wii U) a 64-bit time_t has no lock-free atomic load, so the library needed __atomic_load_8, which the devkitPPC link does not provide. Cobalt's .wuhb build failed to link once its sign-in began resolving DIDs (cobalt#202). The TTLs are now plain values guarded by the cache's existing lock.

Verification

• cmake --build build-wiiu --target wolfram (the Wii U library) builds with no undefined references.
• Host ctest: 152 of 153 pass. The one not run is moderation_demo, an example binary this build directory does not build; it is not touched by this change.

Docs

Docs: none (an internal synchronisation change; no documented behaviour changed). CHANGELOG entry under Fixed.

Changelog: CHANGELOG.md

Checklist

• CI is green (CI gate and flow / conventions)
• No secrets, tokens or signing keys in the diff
• Tests added or changed with the behaviour; none skipped, disabled or deleted

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