fix: use prior FableLoom shots as references

[atomantic]

Summary

• treat a rendered predecessor as a visual reference for FableLoom scene continuity instead of an init image to edit
• allow JSON image-generation callers to name existing gallery images as bounded reference inputs
• preserve honest fallback behavior for missing or unsupported reference conditioning

Test plan

• server: 207 focused image route/provider tests, plus 13 API catalog tests
• client: 27 focused FableLoom/API tests
• client: npm run lint
• client: npm run build
• regenerated API route and socket catalogs