feat: add GetCached method to report cache hit status

[toller892]

Summary

Adds a GetCached method to Group that returns whether the value was served from the local cache, addressing #61.

Motivation

Currently, Group.Get provides no way for callers to distinguish cache hits from cache misses. The only workaround is to compare Stats.CacheHits before and after the call, which is awkward and not goroutine-safe.

Implementation

• New exported method: GetCached(ctx, key, dest) (cached bool, err error)
• Internally refactors Get to delegate to a private getCached helper
• Get behavior is 100% unchanged — this is purely additive
• The cached return value is true only when the value was found in mainCache or hotCache

API

// GetCached acts like Get but additionally reports whether the
// value was served from the local cache.
func (g *Group) GetCached(ctx context.Context, key string, dest Sink) (cached bool, err error)

Tests

Added TestGetCached covering:

• First call returns cached=false (cache miss)
• Second call returns cached=true (cache hit)
• Nil dest returns error

All existing tests pass unchanged.

Fixes #61

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