[havs-mc]

Description

Each attribute, entity and property requirement of a specification calls model.getItemsData for the same applicable items. Each call has a large fixed cost in the fragments worker. On a 15 MB model, one call took ~4 s, and 10 calls of 200 items took 45 s against 7.5 s for one call of 2 000. A specification with many property requirements spends nearly all its time re-fetching.

• IDSSpecification.test() opens a shared scope for its applicable items. Requirements fetch each kind of item data once (attributes; property-set relations) and share it. Outside a specification run, facets fetch as before.
• IDSProperty.getPsets() no longer modifies the fetched occurrence sets when adding type properties. It works on a copy, since the data is now shared.
• IDSProperty.test()'s fetch no longer follows an element's type back to its other instances. In getItemsData, a relation without its own entry takes the config of the relation it was reached through. So ObjectTypeOf under the type inherited IsDefinedBy's relations: true and returned every instance of the type with all their property sets. It is now listed as { attributes: false, relations: false }, as DefinesOccurrence already was. On a 9 600-object model with shared types, one such fetch ran for over 8 minutes without finishing.

Additional context

• On a real 15 MB structural model, a specification with ~30 property requirements went from ~100 s to ~8 s with the equivalent local patch.
• The buildingSMART implementer test cases give the same result as on main (231 of 307 agree).
• New specification-items-data.test.ts: five requirements make two full fetches instead of five, and the results equal those of each requirement run on its own. No fetched item data contains ObjectTypeOf; that test fails without the config change.
• New API surface: IDSFacet.startSharedItemsData / endSharedItemsData (static) and the protected getSharedItemsData. The cache is a module-level WeakMap keyed on the applicable-items object of one run, so separate Components instances don't share anything. Happy to make these internal if you prefer another shape, e.g. passing a cache in the config argument of test().
• Split from the fixes discussed in IDS conformance: status against buildingSMART's implementer test cases, and what is left #831, as suggested there.

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other (performance)

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• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
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• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves.
• Ideally, include relevant tests that fail without this PR but pass with it.

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