tecnickcom / gogen / 28711809731

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feat(sfcache)!: typed generic cache with functional options, stale-if-error resilience, and correctness hardening

Significant improvements to pkg/sfcache, with matching updates to its consumers pkg/dnscache and pkg/awssecretcache.

API:
- Generic value type: Cache[K, V] and LookupFunc[K, V]; Lookup returns (V, error) with compile-time type safety, removing consumer type assertions, the typed-nil-boxing hazard, and the interface boxing allocation on stores.
- Functional options on New: WithTTLFunc computes per-entry TTLs from the key and value; WithStaleIfError serves the last known good value (bounded by maxStale past expiration) when a refresh fails, with automatic recovery on the first success.
- New exported API: ErrNilLookupFunc (a nil lookupFn installs a default failing function instead of panicking), ErrLookupAborted (context-ended waits, wraps the context error), and PurgeExpired.

Correctness fixes:
- Single-flight is preserved when an entry is removed mid-wait (Remove, Reset, or a panicking lookup): waiters re-evaluate cache state instead of spawning duplicate flights, and a stale flight can never clobber a successor's placeholder.
- TTLs use the monotonic clock (immune to NTP steps); extreme TTLs saturate instead of overflowing into an already-expired deadline.
- Capacity overshoot from concurrent distinct-key bursts is reclaimed (previously the map stayed pinned at the burst high-water mark forever), including on the stale-revive path.
- A panic on an unhashable key (possible with interface-typed keys) no longer leaks the read lock and wedges the cache.
- Producer context cancellation no longer poisons coalesced waiters, and only errors that actually are the producer's context error trigger the silent retry; genuine upstream errors are shared.
- The stale window takes precedence over the context-induced retry, so an upstream that hangs (every refresh dying by caller timeout) is still served stale; repeated failures cannot extend the origin... (continued)

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