Refine E2E feature matrix with explicit capability coverage

[rmanibus]

Summary

The E2E suite now has a shared harness, a small compatibility smoke matrix, and feature-oriented tests. The next step is to make feature coverage more explicit and intentional across source/store combinations.

Goals

• Define explicit backend capabilities instead of ad hoc feature filters
• Document which features should run on the full matrix versus representative backends
• Keep the smoke matrix focused on interoperability while feature specs validate product behavior
• Add guidance for placing future tests so the suite stays readable

Notes

PR #172 fixed eager backend/container startup in the old matrix probe loop. The new feature-matrix runner preserves that behavior by keeping backend metadata separate from factory invocation.