Rewrite validation-codes.md at the API-consumer altitude

[JeroenDeDauw]

Follows-up to #1082

A full text pass over the validation-codes reference for its actual reader: an API consumer deciding
what a code means, whether their write fails, and what to do next. Everything that derived or
justified the contract instead of stating it is gone:

• Emitter-class names and validator-loop narration (SubjectValidator, StatementListBuilder, ...).
  Entries now use schema type names (text, dateTime) instead of PHP class names.
• The relation-target resolution anatomy (revision slots, subject-to-page index, graph staleness,
  Imported revisions do not update the graph projection #1022): read consistency is a system-wide property homed in operations/maintenance.md, not
  something to restate per validation code.
• Design-rationale asides and roadmap notes ("would be redundant noise", the 404-asymmetry
  reconciliation note).
• The "Known limitations" inventory of which class performs which check — it duplicated the entries.
• RedHerb's invalid-color entry: extension codes belong to the extension's docs (step 5 of the
  plugin-author section now says exactly that); RedHerb stays as the worked-example pointer.

Blocking semantics now live in one "Blocking and enforcement" section instead of an intro bullet
plus per-entry restatements; only the three non-blocking codes carry a marker, and per-entry
propertyName/valuePartIndex lines that restated defaults are gone.

Completeness fixes found on the way: min-length, max-length, invalid-date, and
unregistered-type are emitted but were undocumented (the last was even referenced by name), and
min-value/max-value are also emitted by date properties, which the emitter list omitted.

_{AI-authored — Claude Code, Fable 5 (max); text-review pass directed by @JeroenDeDauw, spec calibrated over three rounds of in-session discussion of #1082's doc text, the rewrite itself unredirected; not yet human-reviewed; every stated behavior verified against the validator source, doc links checked against their targets, docs-only change.}