feat: add internal BigSegmentsBuilder + config struct

[beekld]

Summary

Internal-only plumbing for Big Segments configuration: a private BigSegmentsBuilder (under src/config/builders/) that produces a public built::BigSegmentsConfig. No ConfigBuilder method yet, so this isn't reachable from customer code.

Test plan

• 9/9 new BigSegmentsBuilderTest cases pass

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Note

Low Risk
New internal config types and builder only; no client wiring or evaluation behavior changes yet, with behavior locked by unit tests.

Overview
Adds internal Big Segments configuration plumbing: a public built::BigSegmentsConfig value type and a private BigSegmentsBuilder under src/config/builders/ that resolves store pointer, LRU cache size/TTL, metadata poll interval, and stale-after threshold.

Build() applies spec defaults (e.g. 1000 cache entries, 5s cache/poll, 2min stale), coerces zero/negative durations back to defaults, and clamps status_poll_interval to stale_after when poll would be slower than staleness detection. A null store is allowed for “not configured.” CMake picks up the new built header glob and big_segments_builder.cpp; nine unit tests cover defaults, setters, coercion, clamping, and repeatability. There is still no ConfigBuilder hook—customer-facing wiring is not in this diff.

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