Adds Pick(origin, direction, candidates, skipServerGuid, maxDistance)
to AcDream.Core.Selection.WorldPicker. Iterates candidates, skips
entities with ServerGuid==0 (atlas/dat-hydrated statics — no server
identity) and the caller's skipServerGuid (the player self).
Geometric ray-sphere intersection at 5m radius (matches
WorldEntity.DefaultAabbRadius). Returns the nearest hit's ServerGuid
within maxDistance (50m default), or null on miss.
6 xUnit tests added: hit, miss, two-in-line-returns-closer, skip-guid,
skip-zero-server-guid, beyond-max-distance.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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