The file has been written with full coverage of all 9 requested symptoms:
- Builder iframe blank — asset 404, vendor:publish fix, CSP/APP_URL/sandbox table
- 419 Page Expired — CSRF meta tag, VerifyCsrfToken middleware, session expiry
- Modules palette empty — context mismatch vs. registration timing, tinker diagnostic
- Notifications not showing — parent::saveStructure() delegation, panel notifications() config
- Media gallery not opening — withMediaGallery() on plugin instance
- Form table columns missing — wrong slug (with tinker cross-check), Table tab not configured, FilamentFormFilters parity
- Reactivity not available — registration timing, must be in boot(), overwrite warning
- 403 on builder page — auth guard mismatch, policy check, temporary authorize() override for isolation
- Octane state leaks — scoped binding, requestTerminated flush, concurrent-load warning