Harden namespace registration plus serializer, validator, and builder cleanups

[amateescu]

Changes

• NamespaceManager::add() now throws on any prefix conflict, including the prov/xsd built-ins, closing a silent-override hole; add addOrReplace() for the deserializer/serializer/ProvGraph path where the container is the authority on its own declarations.
• Add QualifiedName::blankNode(string) for explicit blank-node labels and Attributes::merge() (multimap union, promoting a single value to several under a shared key).
• DocumentBuilder(iterable $namespaces) constructor and Prov::documentBuilder(iterable) to preload namespaces.
• #[\NoDiscard] on Prov::validate() and Prov::deserialize().
• Literal::int() rejects values outside the 32-bit range; PROV-N and PROV-XML emit xsd:long for out-of-range PHP ints.
• ConstraintViolationList implements IteratorAggregate.
• Consolidate the relation endpoint walkers onto RelationMetadata::refEndpoints()/dictionaryEntities(); ProvGraph::referencedIdentifiers() delegates to them.
• JSON serializer: omit an empty prefix map (an empty document stays {}); throw on duplicate bundle identifiers.
• XML serializer: reversible digit-prefix element-name escaping so a genuine _0foo attribute round-trips.
• DocumentComparator: normalize raw-array dictionary key datatypes to full URIs.
• RecordBuilder::resolveAttributes() returns the Attributes::empty() singleton; document merge() concatenation semantics; reword the RelationMetadata::FORMALS docblock.
• Remove ProvNamespace::urnUuid() (a consumer-specific minting helper); URN resolution itself is unchanged.
• Tooling: PHPUnit random execution order + cache directory, explicit ext-dom in require-dev, and scope the mago imprecise-type suppression to the four classes that need it.
• Tests: namespace strictness/addOrReplace, blank-node factory, attribute merge, int-range typing, comparator datatype normalization, violation-list iteration, JSON/XML malformed-input fuzzing, a validator reorder-stability harness, constraint 33/34 boundary directions, ProvGraph default-prefix and built-in-redeclaration cases, and property-pool entries for escapable punctuation and slash/versioned/URN identifiers; tighten the round-trip census so PROV-N/JSON rejection no longer counts as a pass.
• README: PROV-N grammar-extension and identifier-drop notes, DI guidance, and the kept helpers folded into the builder tips.