Add migrations-showcase example and fix false literal defaults

Two changes that together produce a rich, replayable 20-migration
history for migration-aware tooling:

- Fix contract canonicalization dropping literal false / empty-array
  column default values. isDefaultValue() treats false and [] as
  omittable shape defaults, but a default literal payload is data —
  dropping it produced a contract.json that failed its own structural
  validation on the next read (surfaced as PN-CLI-4003 when planning a
  migration for a `Boolean @default(false)` column).
- Add examples/migrations-showcase: 20 cumulative schema snapshots of a
  project tracker (models, enums, relations, indexes, uniques, plus a
  destructive column drop and a retired model) and a generate script
  that runs the real emit → migration plan → migrate flow per step
  against a Postgres database, then captures prisma_contract.marker and
  prisma_contract.ledger (including contract_json_before/after) into
  fixture/prisma-contract.json for downstream demos.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: Søren Bramer Schmidt <sorenbs@gmail.com>

Author: sorenbs

examples/migrations-showcase/README.md

@@ -0,0 +1,15 @@
+# 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.