docs: document marketplace API contracts

[roncodes]

Summary

• document network-store filtering, sorting, distance, pagination, and merchant lookup
• document network-scoped categories, products, and store locations
• document cart store/location metadata, multi-origin service quotes, and checkout customer-token rules
• make collection linting path-safe and replace hundreds of per-file subprocesses with one collection scan

Validation

• npm run postman:lint
• 5 collections and 3 environments validated
• 288 requests scanned with 0 errors and 0 warnings
• git diff --check

Cross-repository dependencies

These contracts describe the Storefront backend changes on dev-v0.4.19, the SDK helpers on feature/marketplace-sdk, and the app behavior on feature/marketplace-edition.

Review focus

Please compare query names, authentication headers, validation behavior, and multi-origin payloads against the backend PR before merging.