BatchedEnrichmentStage buffers events in std::deque members, which allocate and free on each push/pop. Its buffers are bounded by the stage's configured limits, so a preallocated fixed-capacity ring buffer would avoid that per-element allocation churn on a hot path.

Introduce a reusable, self-contained RingBuffer<T> primitive:

• New module src/mongo/db/query/util/ring_buffer.h (+ BUILD.bazel target) with unit tests.
• Fixed capacity set at construction; push / pop / front / size / empty by slot offset, no per-op allocation.
• No production consumers in this ticket; the primitive lands and is unit-tested in isolation.

Built primarily for BatchedEnrichmentStage (adoption tracked separately), but kept general so other bounded-buffer stages can reuse it.

No behaviour change.