Add RevenueCat app catalog integration

[snowmead]

Summary

• Phase 1 CatalogResource for revenuecat/app (Wave 7).
• Registry row + providers/revenuecat module, gap + hermetic provision tests.
• Provisional OUTPUT_FIELDS (api_key) until mise run discover revenuecat/app.
• Branched from refresh tip 30df3de (plugin v0.25.0 fixtures). No smoke fixture.

Test plan

• cargo nextest run -p stackless-integrations -E 'test(revenuecat)'
• registry uniqueness + catalog_outputs_match_output_fields
• mise run check (pre-push hook)

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Note

Low Risk
Additive provider registration and tests only; no changes to auth, provisioning core, or existing integrations.

Overview
Adds RevenueCat as a managed Stripe Projects integration for catalog service revenuecat/app, so stacks can provision it and wire ${integrations.<name>.api_key} into service env.

A new providers/revenuecat module defines RevenuecatApp (global-only config, resource kind integration-revenuecat, single api_key output). OUTPUT_FIELDS are marked provisional until mise run discover revenuecat/app. The provider is registered in registry.rs and included in the catalog_outputs_match_output_fields guard.

Tests cover catalog verification against the shared fixture, a hermetic provision path that asserts api_key is recorded, and registry/output alignment.

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