Fix ordered Multiple field producing duplicate RDF list heads

[Copilot]

ui:Multiple with ui:ordered true produces multiple disconnected list-head triples (subject property _:b1, _:b2, _:b3) instead of a single proper RDF Collection, breaking Turtle serialization and any list-aware consumer.

Root cause

Two bugs in saveListThenRefresh in src/widgets/forms.js:

1. Stale object-index after in-place mutation

rdflib's IndexedFormula indexes Collection objects by element content (ExtendedTermFactory.id), not object identity. addItem/deleteThisItem/moveThisItem all mutate list.elements after the Collection is already in the store. The object-index entry is then stale (keyed under the old element content). The subsequent removeMany() call computes the current (post-mutation) content key, finds no index entry, and throws "Statement to be removed is not on store".

2. Duplicate heads on re-fetch

After putBack invalidates the fetcher cache, updateMany re-fetches the document. The N3 parser creates a new Collection with a different blank-node ID. kb.holds() compares by blank-node ID (via equals()), so it returns false and adds a second subject property Collection triple — the duplicate head.

Fix

saveListThenRefresh — replace removeMany with a safe remove + fresh Collection pattern:

// Before (broken): removeMany re-looks up by mutated content key → throws
kb.removeMany(subject, property, null, dataDoc)

// After: find via subject/predicate index, remove by reference (skips stale object-index)
const oldStatements = kb.statementsMatching(subject, property, null, dataDoc)
oldStatements.forEach(st => kb.removeStatement(st))
// Rebuild with correct index so kb.holds() recognises re-fetched list as duplicate
list = new $rdf.Collection(currentElements)
kb.add(subject, property, list, dataDoc)

kb.statementsMatching(…, null, …) uses the subject/predicate index, bypassing the stale object-index. kb.removeStatement safely skips a missing object-index entry (rdflib's if (!this.index[p][h]) guard). The fresh Collection is keyed under the current element content, so the next re-fetch call to kb.holds() finds it and suppresses the duplicate.

createListIfNecessary — recover an existing Collection from the store before creating a new one (guards against a second head on live-update/reload).

refresh() — keep list in sync with the store's current Collection after external document reloads.

Tests

Added test/unit/widgets/forms/multiple.test.ts covering: existing Collection recovery, lazy list creation, refresh method exposure, pre-populated list rendering, reload sync, single add, double add, and reload + add deduplication.