perf(FragmentsModels): memoize localId index lookups in properties controller

[rihokirss]

Problem

getItemAttributes and getItemRelations in VirtualPropertiesController resolve the flatbuffer array index with a linear indexOf scan on every call:

const index = this._model.localIdsArray()?.indexOf(localId);

Any bulk read (e.g. getItemsData over all property sets, property panels, relation traversals) becomes O(n²). On a real-world 43 MB model with 121k IFCPROPERTYSET items, reading pset names + property values via getItemsData took 18+ minutes of CPU.

Fix

Memoize a localId → index Map for both localIdsArray and relationsItemsArray. The flatbuffer accessors return a fresh TypedArray view on every call, so the memo is keyed by the underlying buffer identity + length instead of array identity, and rebuilds automatically if the model buffer changes (e.g. after edits regenerate the buffer). Items created via edit requests are absent from the map, so lookups fall through to the existing index === -1 code path unchanged.

Results (real IFC-derived fragments models)

Operation	Before	After
getItemsData over 34k psets + 394k properties (17 MB model)	41 s	2.2 s
getItemsData over 121k psets (43 MB model)	18+ min	~13 s

Query results verified identical before/after on real project filters (element sets compared pairwise).

No API changes; single file touched.