test: add round-trip tests and CI workflow

Add the Phase 4 import -> render -> import round-trip: for each of the six
dialects, parse a CREATE TABLE + foreign key schema into a model, render it
back to DDL, parse the rendered DDL again, and assert the two models are
equal. Comparison is on a normalized model (table and column names, resolved
type names, key/constraint flags, relationships), not raw SQL, since
formatting and identifier quoting legitimately differ. A small in-memory
adapter assembles the DatabaseType the renderer consumes from the importer
output, which the app normally reconstructs via the database.

Primary keys are canonicalized to non-nullable in the comparison: SQL Server
emits its primary key as a table-level constraint and the importer only
forces NOT NULL for inline primary keys, so the flag would otherwise differ
on the round trip although the schemas are equivalent.

Add a GitHub Actions workflow running npm ci + npm test on pushes to main
and on pull requests. Typecheck and lint are intentionally left out for now:
the current codebase does not pass a strict project typecheck or a clean
lint, so gating on them would fail CI on pre-existing, unrelated issues.

.github/workflows/ci.yml

@@ -0,0 +1,18 @@
+name: CI
+
+on:
+  pull_request:
+  push:
+    branches: [main]
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 20
+          cache: npm
+      - run: npm ci
+      - run: npm test