Initial commit: Conflict Checker

Cross-discipline design-contradiction checker for construction drawing
sets. Standalone tool broken out from Iron_Bid; a pipeline stage may
later fold back into Iron_Bid.

Pipeline: PDF->images -> per-sheet assertion extraction -> deterministic
clustering by location -> per-cluster reasoning -> report.
Includes CLI (cli/run_check.py) and web UI (backend/main.py).

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

backend/pipeline/constructability.py

@@ -0,0 +1,35 @@
+"""
+constructability.py - Stage 8: constructability review (LLM).
+
+One whole-set reasoning pass flagging things that are drawn but hard, ambiguous,
+or impossible to build as shown (access/clearance, sequencing, support, routing,
+tolerance, dimension closure, detail gaps). Emits the canonical issue schema.
+Returns [] on failure.
+"""
+
+from typing import Dict, List
+
+from backend import config
+from backend.pipeline._serialize import dumps, slim_clusters, slim_sheets
+from backend.pipeline._stage import call_stage, collect_list, validate_issue
+from backend.prompts import (
+    CONSTRUCTABILITY_SYSTEM_PROMPT,
+    CONSTRUCTABILITY_USER_INSTRUCTION,
+)
+
+
+def constructability_review(sheets: List[Dict], clusters: List[Dict],
+                            conflicts: List[Dict]) -> List[Dict]:
+    parsed = call_stage(
+        CONSTRUCTABILITY_SYSTEM_PROMPT,
+        CONSTRUCTABILITY_USER_INSTRUCTION,
+        subs={
+            "assertions": dumps(slim_sheets(sheets)),
+            "clusters": dumps(slim_clusters(clusters)),
+            "conflicts": dumps(conflicts),
+        },
+        max_tokens=config.CONSTRUCT_MAX_TOKENS,
+    )
+    issues = collect_list(parsed, "issues", lambda c: validate_issue(c, "constructability"))
+    print(f"[Constructability] {len(issues)} constructability issue(s)")
+    return issues