feat(graph): rank impact radius by weighted paths

code_review_graph/constants.py

@@ -2,8 +2,33 @@
 
 from __future__ import annotations
 
+import math
 import os
 
+
+def _bounded_float_env(
+    name: str,
+    default: float,
+    *,
+    lower: float,
+    upper: float,
+) -> float:
+    """Read a finite float strictly inside ``(lower, upper)``.
+
+    Invalid environment configuration falls back to the documented default
+    instead of making graph traversal unbounded or failing during import.
+    """
+    raw = os.environ.get(name)
+    if raw is None:
+        return default
+    try:
+        value = float(raw)
+    except (TypeError, ValueError):
+        return default
+    if not math.isfinite(value) or not lower < value < upper:
+        return default
+    return value
+
 SECURITY_KEYWORDS: frozenset[str] = frozenset({
     "auth", "login", "password", "token", "session", "crypt", "secret",
     "credential", "permission", "sql", "query", "execute", "connect",
@@ -19,5 +44,30 @@
 MAX_BFS_DEPTH = int(os.environ.get("CRG_MAX_BFS_DEPTH", "15"))
 MAX_SEARCH_RESULTS = int(os.environ.get("CRG_MAX_SEARCH_RESULTS", "20"))
 
-# BFS engine: "sql" (SQLite recursive CTE) or "networkx" (Python-side BFS)
+# Impact traversal engine: "sql" (bounded SQLite relaxation) or "networkx".
 BFS_ENGINE = os.environ.get("CRG_BFS_ENGINE", "sql")
+
+# ---------------------------------------------------------------------------
+# Impact-radius scoring
+# ---------------------------------------------------------------------------
+# Each hop multiplies the best score so strongly coupled nodes rank first.
+# These review-risk weights intentionally differ from community-clustering
+# affinity weights.
+IMPACT_EDGE_WEIGHTS: dict[str, float] = {
+    "CALLS": 1.0,
+    "INHERITS": 0.9,
+    "OVERRIDES": 0.9,
+    "IMPLEMENTS": 0.9,
+    "TESTED_BY": 0.7,
+    "REFERENCES": 0.6,
+    "DEPENDS_ON": 0.6,
+    "IMPORTS_FROM": 0.5,
+    "CONTAINS": 0.3,
+}
+IMPACT_DEFAULT_EDGE_WEIGHT = 0.5
+IMPACT_DEPTH_DECAY = _bounded_float_env(
+    "CRG_IMPACT_DEPTH_DECAY", 0.6, lower=0.0, upper=1.0,
+)
+IMPACT_SCORE_FLOOR = _bounded_float_env(
+    "CRG_IMPACT_SCORE_FLOOR", 0.05, lower=0.0, upper=1.0,
+)