IDX-7031 Constrain cache generics with concepts

[dalloriam]

Changes

• Move implementation of template methods inline with their declaration
  • I know this is contrary to how we usually do things, but I feel that in this case it was doing more harm than good to repeat

template<class K,
         class V,
         template<class, class, class> class I,
         template<class, class, class> class E,
         template<class, class, class> class C,
         class SV,
         class SK,
         class KH,
         LockingStrategy L>
bool Cache<K, V, I, E, C, SV, SK, KH, L>::insert_locked(K key, V value)

for every method.

• Add new concepts to constrain cache generics

[Copilot]

Pull request overview

This PR refactors cache/policy templates by moving template method implementations inline in headers and introduces new C++20 concepts to constrain cache generics (keys, hashers, measurement functors, etc.), aiming to provide clearer compile-time diagnostics and tighter API contracts.

Changes:

• Moved many template method definitions (cache + policies + bloom filter utilities) inline into their class declarations.
• Added cachemere::detail::traits concepts (e.g., Key, HasherFor, MeasureFor, etc.) and applied them to cache/policy templates.
• Adjusted composite key hashing for basic strings.

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
include/cachemere/policy/insertion_tinylfu.h	Apply concepts to template params; inline method implementations.
include/cachemere/policy/insertion_always.h	Apply concepts to template params; inline method implementations.
include/cachemere/policy/eviction_segmented_lru.h	Apply concepts to template params; inline method implementations; rename/reshape ticket type.
include/cachemere/policy/eviction_lru.h	Apply concepts to template params; inline method implementations.
include/cachemere/policy/eviction_gdsf.h	Apply concepts and introduce a cost functor concept; inline method implementations.
include/cachemere/policy/detail/hash_mixer.h	Constrain hasher template param; inline implementation.
include/cachemere/policy/detail/counting_bloom_filter.h	Inline implementation and refactor internals (shared index computation).
include/cachemere/policy/detail/bloom_filter.h	Inline implementation; adjust include ordering and add traits include.
include/cachemere/policy/constraint_memory.h	Apply concepts to template params; inline method implementations.
include/cachemere/policy/constraint_count.h	Apply concepts to template params; inline method implementations.
include/cachemere/detail/traits.h	Add new concepts for cache generics and call-site constraints.
include/cachemere/composite_key.h	Adjust string hashing to use c_str() for basic strings.
include/cachemere/cache.h	Constrain cache template params with concepts; move template method implementations inline; modernize SFINAE to requires.

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