Gantry: 7 - Opportunistic TTL-map eviction

[vpatelsj]

Scope

Add bounded eviction for in-memory TTL maps.

Problem

Negative cache and related TTL maps use lazy access-only eviction and can retain entries that are never re-queried.

Evidence:

• internal/gantry/negcache/negcache.go:115
• internal/gantry/negcache/negcache.go:139
• internal/gantry/negcache/negcache.go:157
• internal/gantry/coldstart/coldstart.go:950-991
• internal/gantry/mirror/mirror.go:1716-1744

Changes

• Prefer opportunistic sweeps over background goroutines in library types.
• Add nextSweep state and sweep expired entries on mutation and occasional lookup.
• Add an optional max-size cap where necessary, evicting the oldest or earliest cooldown deadline first.
• Apply to negcache.Cache.
• Fire OnSize after sweeps and mutations.
• Apply the same opportunistic sweep pattern to cold-start honorUntil and mirror provider-failure maps.

Tests

• Expired entries are removed on a later mutation/lookup even when the same digest is not requested again.
• Size callbacks observe post-sweep counts.
• Max-size cap evicts deterministic entries.
• Fake clock tests cover negcache, cold-start honor windows, and mirror provider-failure maps.