feat(integrations): add postalform Stripe catalog provider

[snowmead]

Summary

• Adds the postalform 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

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Note

Low Risk
Additive integration and documentation; provisioning follows the same Stripe catalog path as existing managed providers with no auth or core lifecycle changes.

Overview
Adds a PostalForm Phase 1 catalog integration for postalform/mail: managed CatalogResource with optional webhook_url / workspace_name, api_key output (provisional OUTPUT_FIELDS until discover), registry wiring, and catalog gap + hermetic provision tests. Hostable::OUTPUTS drift coverage is extended to PostalForm in the provider test module.

Also lands wave rollout docs (PROVIDER-WAVES.md, catalog-first decision in DECISIONS.md, link from ADDING-A-PROVIDER.md) and a generate_catalog_integrations.py scaffold for future deployables (PostalForm listed as implemented so regen won’t overwrite it).

SDK: stackless-provider-sdk gains shared int_optional, bool_required, and bool_optional config readers for generated/catalog configs.

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[snowmead]

Landed on main via local merge + union-sort (#49).