Add model-page protocol (v1)

[amcritchie]

Task: https://mcritchie.studio/tasks/model-page-engine-protocol

Adds a reusable, admin-only model-page protocol to the engine, drawn into every consuming app via Studio.routes:

• /models/:model/:id — renders one record as pretty JSON plus a copy/paste rails-console command to reload it
• /models/:model/random — bounces to a random record of that model (fresh sample)

What's here

• Studio::ModelPage (app/models/studio/model_page.rb) — the protocol object. Ships an empty registry; a host opts in per model in an initializer: Studio::ModelPage.register("release", Release, lookup: :slug). Mirrors the admin-models host-registry pattern — the engine never constantizes host models by name; an unknown key raises UnknownModel (rendered 404).
• Studio::ModelsController + app/views/studio/models/show.html.erb (engine theme tokens; reuses the components/copy_button partial). Admin-gated via require_admin.
• Routes added to Studio.routes — random drawn before the :id catch-all so it isn't swallowed as an identifier. Host-level helpers (studio_model_path, studio_model_random_path).
• Bumps 0.11.0 → 0.12.0 + CHANGELOG entry.

Tests

• [unit] test/lib/studio/model_page_test.rb — 10 tests (registry, console command, random, JSON), pure-Ruby with a stub host model.
• [integration] test/integration/model_page_routes_test.rb — routes drawn under the dummy app; random-before-:id ordering asserted against the route table.
• Full bin/release-check green (wired both new files into its suite list).

Adoption (follow-ups, gated on publishing 0.12)

• mcritchie-studio registers release and adds a "Model" link on /deployments/all (the app-local proof of this is already operator-approved).
• turf-monster to follow.

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