Adapter dispatch, ElixirCache integration, docs overhaul

[BobbieBarker]

• Centralized adapter dispatch: All adapter calls now route through SchemaCache.Adapter instead of being scattered across schema_cache.ex and set_lock.ex. Dispatch layer handles API translation for ElixirCache modules automatically.
• ElixirCache auto-detection: Behaviour-chain detection identifies use Cache modules at boot. Redis-backed modules get native set operations via command/1. Real integration tests against elixir_cache (test-only dep) replace fake modules.
• Config simplification: Removed Application.get_env fallback. Adapter is now required as a supervisor opt only.
• Documentation overhaul: Fixed terminology (cache-aside not "read through", SMKES as implementation detail not headline feature), added guides (introduction, installation, basic operations, writing adapters, ElixirCache integration, architecture), removed empty "advanced techniques" section, rewrote README.

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[BobbieBarker]

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