isPremiumName decides on every third-party login whether the player
keeps their name, and none of its rules had a test that fails when the
rule breaks: treating a 429 or 5xx from api.mojang.com as "free",
swapping the free and taken TTLs, flipping the freshness comparison,
answering "free" from an expired taken entry during an outage, or
dropping the clear-at-4096 bound. Each of those leaves a squatter
holding a name its owner has bought, or grows the cache without limit,
with CI green.
TestPremiumNameCache drives isPremiumName against a stub that answers
with a fixed status and counts lookups, and seeds cache entries at chosen
ages. Five mutants of handlers_hasjoined.go, one per rule above, each
fail at least one subtest. It does not test an expired "free" entry
during an outage; what that case should return is still open.
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