docs: address PR #51 review — rarity naming, OPS threshold, truncation invariant

- Add rarity name cross-reference table in Background section mapping PRD
  display names (Replacement/Reserve/Starter/All-Star/MVP/Hall of Fame) to
  codebase names (Common/Bronze/Silver/Gold/Diamond/HoF) with IDs
- Fix T4-2: correct Gold OPS threshold from 0.700 to 0.900 (confirmed in
  rarity_thresholds.py); add note that 0.700 is the Bronze floor
- Fix T4-1: restate truncation invariant as a single precise assertion —
  sum(columns) == 108 - truncated_amount — instead of two independent checks
  that can both pass while the sum is wrong for unrelated reasons

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/REFRACTOR_PHASE2_VALIDATION_SPEC.md

@@ -26,6 +26,21 @@ probability system: 2d6 x 3 columns x 6 rows). Each Refractor tier (T1 through T
 accumulated budget across all four tiers is 4.0 chances, equal to approximately 3.7% of the
 108-chance total (4 / 108 ≈ 0.037).
 
+**Rarity naming cross-reference:** The PRD chapters (`prd-evolution/`) use the player-facing
+display names. The codebase and this spec use the internal names from `rarity_thresholds.py`.
+They map as follows:
+
+| PRD / Display Name | Codebase Name | ID  |
+|---|---|---|
+| Replacement        | Common        | 5   |
+| Reserve            | Bronze        | 4   |
+| Starter            | Silver        | 3   |
+| All-Star           | Gold          | 2   |
+| MVP                | Diamond       | 1   |
+| Hall of Fame       | HoF           | 99  |
+
+All rarity references in this spec use the codebase names.
+
 Rarity IDs in the codebase (from `rarity_thresholds.py`):
 
 | Rarity Name  | ID  |
@@ -74,8 +89,12 @@ Verify:
 **Expected Outcome:**
 
 Sum remains 108 after every boost under non-truncation conditions. Under truncation conditions
-(a column hits 0), the sum is reduced by the truncated amount — the implementation discards the
+(a column hits 0), the final column sum must equal exactly `108 - truncated_amount` — where
-excess rather than redistributing it. No column value falls below 0.
+`truncated_amount` is the portion of the 1.0-chance budget that was dropped due to the 0-floor
+cap. This is a single combined assertion: `sum(columns) == 108 - truncated_amount`. Checking
+"sum <= 108" and "truncated amount was discarded" as two independent conditions is insufficient
+— a test can pass both checks while the sum is wrong for an unrelated reason (e.g., a positive
+column also lost value due to a bug). No column value falls below 0.
 
 **Risk If Failed:**
 
@@ -121,7 +140,9 @@ Worked example for validation reference:
 The cumulative 4.0-chance shift produces a ~3.7% total movement from negative to positive
 outcomes. No single outcome column increases by more than 2.5 chances across the full T4
 journey under any profile. The card remains recognizably Bronze — it does not cross the Gold
-OPS threshold (0.700 for 2024/2025 thresholds) unless it was already near the boundary.
+OPS threshold (0.900 for 2024/2025 thresholds; confirmed in `rarity_thresholds.py`
+`BATTER_THRESHOLDS_2024.gold` and `BATTER_THRESHOLDS_2025.gold`) unless it was already near
+the boundary. Note: 0.700 is the Bronze floor (`bronze` field), not the Gold threshold.
 
 **Risk If Failed:**