feat(showcase): demonstrate a multi-value reference seed (#3911)

`field_zoo` covered every field type with a real seeded value EXCEPT a
multi-value reference — the one shape #3911 was about. Adds
`f_lookups: Field.lookup('showcase_account', { multiple: true })`, seeded from
an array of natural keys (`['Northwind', 'Contoso']`) so each element resolves
independently to an account id.

That makes the specimen complete and doubles as a boot-time regression guard,
which is what field_zoo exists for: before the fix the array was rejected as a
non-string reference and the field vanished from the row, so the gallery
shipped silently missing one relational type.

The multi-value USER field (`f_users`) stays unseeded, now with the reason
recorded on the object: `sys_user` rows come from SIGN-UP, not seeds (see
security/seed-approval-demo.ts), so on a fresh boot no human user exists for a
natural key to resolve against. Authoring one would make the whole showcase
seed load report success: false on every first boot. The multi-value reference
mechanics the two fields share are covered by `f_lookups`.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Gusro3gdGv4wgbBnaFy9ah

Author: claude

examples/app-showcase/src/data/objects/field-zoo.object.ts

@@ -99,10 +99,22 @@ export const FieldZoo = ObjectSchema.create({
 
     // ── Relational ───────────────────────────────────────────────────────
     f_lookup: Field.lookup('showcase_account', { label: 'Lookup → Account' }),
+    // Multi-value reference — stores an ARRAY of account ids (JSON column).
+    // Seeded from an array of natural keys, one per element (framework#3911);
+    // this is the seedable half of the `multiple: true` reference surface —
+    // see `f_users` below for the half that a fresh boot cannot seed.
+    f_lookups: Field.lookup('showcase_account', { label: 'Lookup → Accounts (multiple)', multiple: true }),
     f_master_detail: Field.masterDetail('showcase_project', { label: 'Master-Detail → Project' }),
     f_tree: { type: 'tree', label: 'Tree (self/category)', reference: 'showcase_category' },
 
     // ── User (lookup specialized to sys_user) ────────────────────────────
+    // NOT seeded, and deliberately so: `sys_user` rows are created by SIGN-UP,
+    // not by seeds (see security/seed-approval-demo.ts — "users can't be
+    // seeded (they sign up)"), so on a fresh boot there is no human user for a
+    // natural key to resolve against. Authoring one here would make the whole
+    // showcase seed load report `success: false` on every first boot. The
+    // multi-value REFERENCE mechanics these fields share are demonstrated by
+    // `f_lookups` above; assign these two in the UI once you have signed up.
     f_user: Field.user({ label: 'User → sys_user (single)' }),
     f_users: Field.user({ label: 'Users (multiple)', multiple: true }),
     f_owner: Field.user({ label: 'Owner (current_user default)', defaultValue: 'current_user' }),

examples/app-showcase/src/data/seed/index.ts

@@ -256,6 +256,15 @@ const memberships = defineSeed(ProjectMembership, {
 // Relational/computed fields (lookup/master_detail/tree/record-map, formula/
 // summary/autonumber) resolve or generate at runtime. `f_master_detail` is the
 // owning Project; `f_lookup` the Account — referenced by their seed externalId.
+//
+// `f_lookups` is the MULTI-VALUE reference case (framework#3911): a
+// `multiple: true` lookup stores an array of ids, so the seed value is an
+// ARRAY of natural keys and each element resolves independently. It is also
+// the regression guard for that resolution — before the fix the array was
+// rejected as a non-string reference and the field vanished from the row,
+// leaving the specimen silently missing one relational type.
+// The multi-value USER field (`f_users`) stays unseeded on purpose: sys_user
+// rows come from sign-up, not seeds — see the note on the object.
 const fieldZoo = defineSeed(FieldZoo, {
   mode: 'upsert',
   externalId: 'name',
@@ -272,7 +281,7 @@ const fieldZoo = defineSeed(FieldZoo, {
       f_date: '2026-06-17', f_datetime: '2026-06-17T14:30:00Z', f_time: '14:30',
       f_boolean: true, f_toggle: true,
       f_select: 'high', f_multiselect: ['red', 'green'], f_radio: 'yes', f_checkboxes: ['email', 'push'], f_tags: ['alpha', 'beta'],
-      f_lookup: 'Northwind', f_master_detail: 'Website Relaunch',
+      f_lookup: 'Northwind', f_lookups: ['Northwind', 'Contoso'], f_master_detail: 'Website Relaunch',
       f_location: { lat: 47.6062, lng: -122.3321 }, f_address: { street: '1 Main St', city: 'Seattle', state: 'WA', postal_code: '98101', country: 'US' },
       f_code: '{\n  "ok": true\n}', f_json: { nested: { k: 'v' }, list: [1, 2, 3] }, f_color: '#2563EB',
       f_rating: 4, f_slider: 60, f_progress: 80,