Module workers/data-worker/projection/hidden-dims

Hidden-dimension classification shared by the GSplats projection dispatcher (and its golden-equivalence tests).

nD → 3D projection splits the non-displayed ("hidden") dimensions into three disjoint groups:

• extend_to_all — skipped entirely; the splat is visible across every value of the dimension (tolerance ≥ EXTEND_TO_ALL_THRESHOLD).
• discrete — binary half-step visibility (time, channel, …).
• continuous — shifted-Gaussian Mahalanobis attenuation.

This split was previously inlined in both the worker dispatcher and the (now-deleted) main-thread copy. Extracting it keeps the dispatcher lean and makes the classification independently unit-testable.