Store dateTime statements as Neo4j datetime values

[malberts]

Statement values for the dateTime property type were written to Neo4j as
plain ISO 8601 strings, leaving them un-queryable with Cypher's datetime
functions (comparisons, ordering, range filtering). Builders registered
with Neo4jValueBuilderRegistry can now return a Neo4jTypedValue, which
SubjectUpdater wraps in a Cypher constructor (e.g. datetime()) on save.

The mechanism extends cleanly to other typed Neo4j properties (date,
point, duration, etc.) without further restructuring.

Result of back-and-forth with @malberts. Context: the NeoWiki codebase,
the existing PageDateTime / Neo4jQueryStore::extractTypedValues pattern,
and Neo4j Cypher temporal type docs.
_{Written by Claude Code, Opus 4.7 (1M context)}