test: make cache TTL expiry deterministic

[fxdv]

Root cause

Post-merge main CI for the v0.1.4 VERSION commit failed in TestTTLExpiry: https://github.com/fxdv/patchlog/actions/runs/30047779586

The test used a 100 ms wall-clock TTL. Cache.Set performs an atomic durable write, so a slow CI filesystem could consume the TTL before the immediate Get, producing a false failure.

Fix

• use a one-minute live TTL for the immediate-hit assertion;
• rewrite the serialized test entry with an explicit timestamp two minutes in the past;
• assert expiry without sleeping or depending on scheduler/filesystem latency.

Production cache behavior is unchanged.

Evidence

• TestTTLExpiry, Go 1.26.5, -count=100: green;
• TestTTLExpiry, supported Go 1.22.12, -count=100: green;
• bash scripts/gate.sh: 8/8 green when loopback listeners are allowed.

No v0.1.4 tag was created. Finalization remains blocked until this PR is merged and post-merge main CI is green on the new exact commit.