Fix date-shaped identifier misparsing in PROV-N and silent skipping of malformed PROV-JSON record bodies

[amateescu]

Changes

• ProvNDeserializer: a date-prefixed token matching the full dateTime
  shape (e.g. 2024-01-15T100000) was parsed as a DateTimeImmutable
  whenever it matched that shape, regardless of grammar position. An
  identifier like entity(2024-01-15T100000) reused in wasGeneratedBy's
  entity slot would silently come back as a date instead of an identifier.
  Only the grammar position ($expectTime), not the token shape, can tell
  an identifier from a datetime — the check now gates on that.
• JsonSerializer: add assertRecordBody(). A scalar record body such as
  {"entity":{"ex:e1":"notamap"}} was silently skipped (if (!is_array($instance)) continue;).
  It now throws DeserializationException, matching how other malformed
  PROV-JSON shapes are already handled. Covers entity/activity/agent
  records and both scruffy-list and single-instance relation bodies.
• ProvNSerializer: cosmetic normalization of the lang-tag regex's
  character class ordering; no behavior change.
• ConstraintValidator: doc comment correction — rules 22-23 are
  key/merging rules (unifying records that share an identifier across a
  document), distinct from the transitive graph reasoning rules
  (31-32, 35, 41-49) that are the rest of the unsupported set.
• JsonLdSerializer: doc comment clarifying that a relation with a null
  subject formal is omitted from output, since JSON-LD attaches a relation
  as a property of its subject node.