Persist per-migration contract snapshots into the ledger

examples/migrations-showcase/README.md

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+# migrations-showcase
+
+Generates a realistic 20-migration history with the real Prisma Next migration flow, for demoing migration-aware tooling (e.g. the Prisma Studio Migrations view).
+
+`steps/` holds 20 cumulative snapshots of a project-tracker schema — models, fields, enums, relations, indexes and uniques are added over time, and a few steps are deliberately destructive (a dropped column, a retired model) so downstream diffs exercise every change kind.
+
+## Run it
+
+```bash
+pnpm --filter migrations-showcase generate
+```
+
+For each step the script emits the contract, plans an attested migration bundle into `migrations/app/`, and applies it to an ephemeral Prisma Postgres dev database. Every apply writes one `prisma_contract.ledger` row carrying the migration's `contract_json_before` / `contract_json_after` snapshots.
+
+After the last step the script captures `prisma_contract.marker` and `prisma_contract.ledger` into `fixture/prisma-contract.json`, ready to be replayed by demo seeds.