fix(rank): round weighted scores without binary drift (#588)

* fix(rank): round weighted scores without binary drift

* docs(changelog): record fix for #588

* changelog: place the #588 entry under [Unreleased]

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

---------

Co-authored-by: Mads Lorentzen <madslorentzen_17@hotmail.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

tools/rank_state.py

@@ -45,6 +45,7 @@ import sys
 import tempfile
 import unicodedata
 from datetime import date, datetime, timedelta, timezone
+from fractions import Fraction
 from pathlib import Path
 
 ROOT = Path(__file__).resolve().parent.parent
@@ -282,15 +283,18 @@ def cmd_sweep(args) -> int:
 def overall_score(scores: dict) -> int:
     if not isinstance(scores, dict):
         raise ValueError("scores must be an object")
-    total = 0.0
+    total = Fraction(0)
     for dim, weight in WEIGHTS.items():
         value = scores.get(dim)
         if isinstance(value, bool) or not isinstance(value, (int, float)):
             raise ValueError(f"missing or non-numeric score '{dim}'")
         if not 0 <= value <= 100:
             raise ValueError(f"score '{dim}' must be finite and between 0 and 100")
-        total += float(value) * weight
-    return int(total + 0.5)
+        # Scores and documented percentage weights are decimal quantities.
+        # Binary float summation can turn 59.5 into 59.49999999999999 and drop
+        # an otherwise valid result into the lower verdict band.
+        total += Fraction(str(value)) * Fraction(str(weight))
+    return int(total + Fraction(1, 2))
 
 
 def band(score: int) -> str: