createMatExpansionSetSync

Description#

Set-based bidirectional sync between a <mat-accordion>'s projected <mat-expansion-panel> tree and the headless CngxAccordion open-set. The accordion open-set is a multi-membership Set<string>, so the index-based createMaterialBidirectionalSync (@cngx/common/data) does not fit — this helper syncs per-panel expanded against set membership instead.

Installs two effects and one listener-set:

1. brain→Material — an effect() tracking panels() and, per panel, accordion.isOpen(id). When the open-set (or the panel list) changes it writes panel.expanded to match, guarded by a re-entrancy flag so the resulting synchronous expandedChange echo does not loop back. The Material writes run inside untracked() per reference_signal_architecture rule 2.
2. subscription reconcile — an effect() tracking only panels(); inside untracked() it subscribes freshly-added panels' expandedChange and drops removed ones (diff-only churn, a Map<P, subscription>).
3. Material→brain — each expandedChange listener flips the brain via accordion.toggle(id), but only when the incoming expanded differs from the brain's current (clamped) membership, so a programmatic echo (or a redundant Material emit) is a no-op. Routing through toggle (not a raw set-write) means single-mode arbitration and the exposed openIds model stay identical to the native CngxAccordionPanel DOM path.

The idempotent membership check plus the re-entrancy flag together close the loop: the flag drops the synchronous echo CdkAccordionItem emits from its expanded setter during our own write, and the membership check drops any Material emit that already agrees with the brain.