Document all stable GraphQL validation error markers

Context

#198 adds an important schema stability policy and names Cannot query field as stable validation/error text. Luis' latest review notes that downstream compatibility actually depends on three literal markers:

• Cannot query field
• Unknown argument
• Unknown type

mina-explorer-api matches all three to classify UpstreamSchemaError, drive tier fallback, and poison its capability cache. mina-explorer keys mostly on Cannot query field.

Follow-up work

• Update the policy/docs so all three literal markers are named as stable downstream contract, not only Cannot query field.
• Make it clear that changing any of the three messages, masking them, or replacing them with armor/cost-limit errors can be breaking.
• Consider explicit regression tests for Unknown argument and Unknown type alongside the existing Cannot query field checks.

Acceptance criteria

• docs/versioning.md or the schema stability policy names all three stable markers.
• The docs state that validation/parse masking changes affecting these markers require a major release.
• Regression coverage pins all three marker families, or the gap is explicitly accepted.

Related

• #198
• #183
• #195

Start with docs/versioning.md or the schema stability policy and read #198, then locate the existing checks for the Cannot query field marker. Document Cannot query field, Unknown argument, and Unknown type as stable contract markers, including the major-release rule for masking or changing them. Add regression coverage for all three families, or record an explicit decision to accept the gap.