Performance hot paths plus attribute set semantics, entity involvements, and serializer ordering

[amateescu]

Changes

• Speed up the serialization and namespace-pruning hot paths: a fast path in QualifiedNameEscaper::escape(), a lookup cache on PrefixMinter::prefixFor(), lazy blank-label collection in JsonSerializer, qualified-name string-form reuse, a decode guard on JSON deserialization, and namespace-URI-based pruning in the builders.
• Adopt set semantics for attribute values: Attributes now collapses identical attribute-value pairs in its constructor (PROV-DM models a record's attributes as a set of pairs), guarded so single-valued keys pay nothing. Value identity moves to the new internal ValueIdentity, shared with DocumentComparator so dedup and equality always agree, including the bare-scalar vs typed-Literal cross-type case. Observable only as count() dropping for inputs that carried duplicate pairs.
• Remove the unused Attributes::merge(): it had no production callers, and the multimap is built via with(), from(), and AttributesBuilder.
• Add RecordContainer::entityInvolvements(), a forward pass yielding a public EntityInvolvement (relationType, role, entity) per entity-typed relation endpoint, so a consumer can derive a secondary index from a finished Document or Bundle. A typed derivation reports its subtype (wasRevisionOf etc.), and role distinguishes endpoints such as generatedEntity from usedEntity.
• Add the opt-in RecordBuilder::autoDeclareEntities(): at build() it emits a bare Entity for any entity-typed relation endpoint no record declares, skipping blank nodes and already-declared identifiers and propagating to bundles. Off by default.
• Always sort namespace declarations in every serializer (the prov and xsd built-ins first, then alphabetically by prefix), and add an opt-in sortRecords flag that orders records into PROV-DM concept order (elements, then relations in component order, then by identifier). Ordering is non-semantic in every PROV format, so round-trips and semantic equality are unaffected.