describe emits one canonical reference; compare can anchor on it

Describe mode now produces a single coherent, internally-consistent canonical
scene description (paragraph + per-axis spec, written to canonical_reference in
the report). Compare gains an optional reference_description input: when set, it
anchors on that fixed text and shows only the generated image (no swap) — so the
reference side never drifts or self-contradicts across iterations; only the
generated image is re-described each turn. agent_bridge gains --ref-desc /
--ref-desc-file (reads the describe report's canonical_reference). Docs + example
workflow updated.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

workflow/workflow_api.json

@@ -75,7 +75,8 @@
       "auto_download": true,
       "report_dir": "/media/p5/Comfyui/output/calibrator",
       "run_tag": "",
-      "prompt_used": ""
+      "prompt_used": "",
+      "reference_description": ""
     },
     "_meta": { "title": "Qwen3-VL Image Judge (Calibrator)" }
   }