SelectionState (new) owns the multi-set, the "active" id (last single-
clicked), and a per-object_id flags SSBO bound at binding=3. Main
shader reads sel_flags[v_object_id] for the in-set tint and a separate
u_active_id uniform for a stronger tint on the active.
Click semantics: plain replaces, Shift/Ctrl toggles. LMB-drag past 5px
boxes the rect through a pick-pass readback — plain replaces, Shift
adds, Ctrl removes; box-select preserves the active. Drag promotes
regardless of start point so a press on geometry doesn't disqualify it.
Sidecar fast-path bulk-loads instances, so noteObjectId is also called
from the apply path — without it the flags buffer was sized to 1 slot
while object_ids were in the 100k+ range and the in-set bit was lost.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
Dion Moult