feat(ml): CCIP character matches ground to the matched figure region (#133 step 2)

match_image now tracks WHICH query figure produced the winning cosine per
character (argmax over the per-figure best-reference sim) and attaches its bbox as
grounding {bbox,kind:'figure',detector}. SuggestionService carries it: a CCIP-only
character hit grounds to its figure; a 'both' hit keeps the head's localized crop
if it had one, else falls back to the CCIP figure — so corroborated characters
stay grounded. Test: a character match carries the matched figure's bbox+kind.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

backend/app/services/ml/suggestions.py

@@ -116,9 +116,13 @@ class SuggestionService:
             if ex is not None:
                 ex["source"] = "both"
                 ex["score"] = max(ex["score"], c["score"])
+                # Keep the head's localized crop if it had one; else fall back to
+                # the CCIP figure so a corroborated character still grounds (#1206).
+                ex["grounding"] = ex.get("grounding") or c.get("grounding")
             else:
                 merged[key] = {
                     "name": c["name"], "score": c["score"], "source": "ccip",
+                    "grounding": c.get("grounding"),
                 }
 
         result = SuggestionList()