feat: comprehensive Elixir patterns guide with source citations
25 patterns extracted from the Elixir standard library source code, each with exact file:line citations for authoritative reference. Covers: multi-clause dispatch, type specialization, tagged tuples, protocols, use/__using__, behaviours, binary parsing, supervision, GenServer patterns, Agent wrappers, options validation, pipe operator, Enumerable/Collectable protocols, Task async/await, documentation conventions, parameterized testing, error messages, defoverridable, infinity sentinels, defguard, Erlang delegation, flexible APIs, and naming conventions.
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