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>

docs/CALIBRATION_POLICY.md

@@ -47,18 +47,21 @@ pose cluster is split into many axes so the agent gets specific, actionable targ
 ## Step 0 — first pass (describe / bootstrap)
 
 The very first iteration has no generated image yet, so the judge runs in **describe
-mode**: it looks at the reference alone and returns a prompt-ready `caption` plus a
-per-axis target spec. That seeds everything:
+mode**: it looks at the reference alone and emits **one canonical scene description** —
+a coherent, internally-consistent paragraph plus a per-axis target spec. That seeds
+everything *and* becomes the fixed reference for the whole loop:
 
 ```bash
 python agent_bridge.py --mode describe --workflow workflow/workflow_describe_api.json \
   --run-tag seed --analysis-dir <report_dir>
 ```
-→ `latest.json` = `{"mode":"describe", "caption":"...", "axes":{axis: "value", ...}}`
+→ `calib_seed.json` = `{"mode":"describe", "description":"…", "axes":{axis:value,…}, "canonical_reference":"…"}`
 
-The agent takes `caption` as the **initial prompt** and `axes` as the **initial
-axis_state**, then enters the compare loop below. No reference description has to be
-written by hand — the VL provides the target to reproduce.
+The agent takes `description` as the **initial prompt** and `axes` as the **initial
+axis_state**. Crucially, the compare loop then **anchors on this canonical reference**
+(via `--ref-desc-file`) instead of re-reading the reference image every iteration — so the
+`ref` side never drifts or contradicts itself across passes; only the generated image is
+re-described each turn.
 
 ## Per-iteration algorithm (greedy per-axis hill-climb)
 
@@ -69,6 +72,7 @@ loop:
   prompt = render(state)                       # state = current value per axis
   report = run agent_bridge.py --prompt prompt --negative state.negative
                                --seed state.seed --run-tag iter{i}
+                               --ref-desc-file <report_dir>/calib_seed.json   # anchor on canonical ref
                                --workflow wf.json --analysis-dir <report_dir>
   if report.mismatch_count == 0 and report.overall_score >= TARGET:
       stop("converged", state)                 # TARGET e.g. 0.9 (mostly match)