multidict is a high-performance, dual-implementation (C extension and pure Python) multi-value mapping library for Python, designed primarily for HTTP headers and URL query strings. It provides `MultiDict`, `CIMultiDict` (case-insensitive), read-only proxy variants, and the `istr` case-insensitive string subclass. The C extension, built with hand-written C and aggressive optimization, delivers O(1) lookups, insertion-order preservation, and thread-safe lock-free readers under CPython's free-threading mode. The public C API capsule enables other C extensions to create and manipulate multidicts directly. The codebase exhibits exceptional engineering discipline: behavioral parity between the two implementations, extensive test coverage including property-based (Hypothesis), concurrency stress, memory leak, and benchmark regression tests, plus a structured changelog with towncrier. While tightly coupled to CPython internals and carrying significant maintenance burden from free-threading support, the core data structure patterns and concurrency primitives are elegantly designed and potentially reusable in other C extension projects.