Multi-schema-version reading to make require_major_on_schema_change reachable

[charles-fineman]

Context: Declared forward-looking contract, documented in docs/cli.md and inline in internal/continuity/continuity.go; flagged by architectural review of #1.

compare validates both snapshots first, and each tool release reads exactly one schema version per document type, so a schema-version difference can never reach the require_major_on_schema_change transition rule through the CLI today. The rule takes effect with the first tool release that reads more than one schema version (the planned migration mechanism in docs/versioning.md).

Acceptance criteria:

• When the first schema revision lands, Validate accepts a defined window of schema versions (e.g. N and N-1) so compare can genuinely observe a transition, or the rule is re-scoped with an explicit decision record.
• End-to-end test drives the rule through the CLI once reachable.