Ports retail CELLARRAY::add_cell (acclient_2013_pseudo_c.txt:701036): ordered list,
dedup by cell_id, append at end. The order is load-bearing for the verbatim
find_cell_list current-cell-first interior-wins pick (next commits) that fixes the
R1 cottage membership flap. Implements ICollection<uint> (helper-facing) +
IReadOnlyCollection<uint> (consumer-facing). 5 unit tests.
Also lands the membership-port pseudocode (workflow step 3) + the Stage-1 plan.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
