Verify gate for the generated migration fragment

docs/manual/_generated/supported-schema-shapes.adoc is a committed materialized view over the classifier traces (LeafCoverageReport, migration mode), but unlike its sibling supported-directives.adoc it has no --verify execution: the CI leaf-coverage step regenerates a report artifact without comparing the committed fragment, so a regeneration from a partial trace set commits silently. This has bitten twice in one programme, in both directions (a shipped shape advertised as (not yet supported) in the user manual’s migration page, and the inverse), and each time the error was caught by hand at a review gate rather than by the build. Add a verify-mode execution for the migration fragment so a full-trace mismatch fails the build the same way the directive-support fragment already does, and decide where it runs (the trunk-gated CI step has full traces; a local -pl build does not, so the gate needs the same no-traces short-circuit the tool already carries).