[IDX-7035] Misc. API improvements

[dalloriam]

Changes

• Add Cache::find_or_insert() (src)
• Use <=> operator for comparing keys/key views when possible (src)
• Add CompositeKey (src)

[Copilot]

Pull request overview

This PR improves the cache API and heterogeneous lookup support by adding a reusable composite key type, extending transparent equality to support three-way comparisons, and introducing a find_or_insert convenience API with accompanying tests.

Changes:

• Add cachemere::CompositeKey<...> with Abseil hashing support (including consistent hashing for std::string vs std::string_view).
• Extend detail::TransparentEq to fall back to <=> when == is not available, enabling more heterogeneous comparison patterns.
• Add Cache::find_or_insert API and tests validating the insertion/caching behavior.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
tests/src/detail/heterogeneous_lookup_tests.cpp	Adds coverage for heterogeneous comparisons using <=>.
tests/src/detail/composite_key_tests.cpp	New tests for CompositeKey construction, comparison, hashing, and heterogeneous lookup in a cache.
tests/src/cache_tests.cpp	Adds test ensuring find_or_insert only calls the factory once and caches the result.
tests/CMakeLists.txt	Registers the new composite key test file in the test target.
include/cachemere/detail/transparent_eq.h	Adds <=>-based equality fallback for transparent comparisons.
include/cachemere/detail/traits.h	Adds string/string_view traits and a FactoryFn concept used for find_or_insert.
include/cachemere/composite_key.h	Introduces CompositeKey with tuple-based storage and Abseil hashing.
include/cachemere/cache.h	Adds find_or_insert API and implementation.
include/cachemere.h	Exposes composite_key.h from the umbrella header.
CMakeLists.txt	Adds composite_key.h to the interface sources.

Comments suppressed due to low confidence (1)

include/cachemere/detail/traits.h:51

• FactoryFn's template parameter order (FactoryFn<T, K, V>) doesn't match how it's being used in cache.h (where Fn is the callable). Consider redefining the concept as template<typename Fn, typename K, typename V> concept FactoryFn = ... and use const K& (and possibly std::invoke) in the requires-expression so lambdas/functions match naturally without requiring copies.

template<typename T, typename K, typename V>
concept FactoryFn = requires(T t, K key) {
    { t(key) } -> std::convertible_to<V>;
};

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