Improve CacheStore correctness, add cache() helper

[techmahedy]

This PR strengthens Doppar’s cache layer, adds a new cache() helper.

What Changed

Caching implementation:

• tightened CacheStore key validation and iterable handling
• made clear() namespace/prefix-scoped instead of clearing unrelated cache entries
• improved stash() and stashForever() so cached null values are treated as valid hits
• added synchronized mutation paths for add(), increment(), and decrement()
• introduced IncrementableCacheInterface to make the rate limiter depend on the methods it actually uses
• updated RateLimiter to use add()/increment() more safely and predictably
• registered the new cache contract in the cache service provider

Helpers:

• added global cache() helper support for:
• cache() to resolve the cache store
• cache('key', $default) to read values
• cache(['key' => 'value'], $ttl) to write values
• cache()->... object-style chaining

Tests:

expanded cache tests for:

• traversable inputs in bulk operations
• prefix-scoped clearing
• cached null behavior in stash helpers
• stricter key validation
• helper usage