Move V_delta schema_version from base to document_class

[stevevanhooser]

Summary

Companion to did-schema #55. schema_version is the version of the overarching schema set (V_delta vs did_v1), not a payload field on any class — so it lives at document_class.schema_version (sibling of class_name, class_version, superclasses), never on base.

The previous placement (base.schema_version, a declared field on V_delta's base.json) is what caused the ~16 DID:Database:ValidationFields errors on NDI-matlab #803 after #140 cleared the superclass-chain mismatch: NDI's legacy base_schema.json declares only session_id/id/name/datestamp under base, and the unknown 5th base field tripped the validator. Moving the tag to document_class removes it from the base block entirely — NDI's legacy validator sees no extra field; the V_delta strict-fields check (cache.m:446-458) only enforces declared fields on chain blocks, not on document_class, so the new structural key is accepted without further change.

Changes

• universalRenames stamps document_class.schema_version instead of base.schema_version. Also migrates a stale base.schema_version forward (bodies emitted by an earlier V_delta-draft migrator carry the tag on base; the new pass moves it to document_class and strips from base) so the strict-fields validator on the next write doesn't reject the legacy placement.
• v1_to_v2.isAlreadyVDelta short-circuit reads from document_class.schema_version.
• cache.buildBlankDocument stamps document_class.schema_version = 'V_delta' so freshly built blanks are immediately recognised as V_delta-shaped.
• Tests: makeVDeltaSkeleton puts schema_version on document_class; existing assertions on doc.get('base.schema_version') rewritten to doc.get('document_class.schema_version'). Three new assertions: stale-base→document_class migration, blank-document stamp, and absence of base.schema_version after migration.

Merge order

1. did-schema Documents and document data should be permanently removed when they are removed from all branches #55 (merged ✓)
2. this PR (CI fetches did-schema@main, which now lacks schema_version from base fields)
3. NDI-matlab #803 re-run picks both up via the dependency fetch and clears the remaining 16 DID:Database:ValidationFields errors. (The testTuningCurveCalc numeric failure is independent.)

Test plan

• did-matlab CI green on this PR (unit tests including the new schema_version-placement assertions, end-to-end migrate-then-validate path)
• After merge, kick NDI-matlab #803 CI and confirm the 16 structural failures clear

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