Revisit BrainCollection for 0.6.1 against stabilized member contracts

BrainCollection and its exclusive support were deferred from 0.6.0 to 0.6.1 while every other
planned 0.6.0 feature was retained.

Preserve the implementation reference at 5b0080be and
development/specs/braincollection.md. Before reintroduction, reconcile member ownership,
cached/in-memory equivalence, fit/predict/contrast semantics, archive versus image export,
cache lifetime and memory/concurrency with the stabilized BrainData, Glm, Ridge and
DesignMatrix.

Do not revive historical pipeline abstractions or automatically copy the old execution layer.
Reuse proven shared internals. Bring a design and release-scope proposal for approval before
implementation. No start date.

Note: development/execution-model.md preserves the deferred 0.6.1 execution design; it is not
an active 0.6.0 subsystem.

Start with development/specs/braincollection.md and implementation reference 5b0080be, then compare the stated contracts with BrainData, Glm, Ridge and DesignMatrix. Prepare a design and 0.6.1 release-scope proposal covering ownership, equivalence, semantics, export, caching, memory and concurrency; the issue is done when that proposal is approved.

Written by the indexing model from the issue text.