feat(integrations): add mixpanel Stripe catalog provider

[snowmead]

Summary

• Adds the mixpanel Stripe Projects catalog integration family as CatalogResource adapter(s).
• Follows docs/ADDING-A-PROVIDER.md and docs/PROVIDER-WAVES.md.
• Includes gap test + hermetic provision test. OUTPUT_FIELDS provisional until mise run discover.

Test plan

• Gap + hermetic tests in module
• Live mise run discover when provider is linked
• Merge after resolving additive registry.rs conflict with other family PRs

---

Note

Low Risk
Changes follow the established CatalogResource pattern with offline tests only; provisional output fields may need discover before live smoke.

Overview
Adds a Mixpanel Phase 1 catalog integration for mixpanel/analytics: managed CatalogResource with empty config, provisional TOKEN → token output mapping, registry wiring, and catalog gap + hermetic provision tests. Hostable::OUTPUTS vs OUTPUT_FIELDS drift is guarded for Mixpanel in the existing provider test.

Also extends stackless-provider-sdk with optional/required boolean and optional integer config helpers for generated catalog modules, documents catalog-first waves (PROVIDER-WAVES.md, DECISIONS.md, link from ADDING-A-PROVIDER.md), and adds scripts/generate_catalog_integrations.py to scaffold remaining deployables without rewriting registry.rs.

^{Reviewed by Cursor Bugbot for commit a0b561d. Bugbot is set up for automated code reviews on this repo. Configure here.}

[cursor]

Cursor Bugbot has reviewed your changes using default effort and found 2 potential issues.

^{❌ Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, enable autofix in the Cursor dashboard.}

^{Reviewed by Cursor Bugbot for commit 1800731. Configure here.}