fix(rules): only total concealment blocks sneak attack

PF1e (CRB rogue Sneak Attack): "A rogue cannot sneak attack while
striking a creature with total concealment." _sneak_attack_dice now gates
on miss_chance >= 50 instead of any concealment > 0, so a blurred (20%)
target can still be sneak attacked while a displaced (50%) one cannot. The
test that pinned the old any-concealment denial was retargeted to total
concealment and a partial-concealment case added; README updated.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/pf1e_simulator/combat.py

@@ -2496,12 +2496,17 @@ class CombatEngine:
 
         PF1e: precision damage when the target is flanked (melee only, melee
         touch included) or denied its Dex bonus to AC (melee, or ranged within
-        30 ft); dice = (level + 1) // 2; concealment denies the sneak attack
-        entirely. PF1e (CRB): undead and constructs are immune to precision
-        damage — ooze/plant immunity is not modeled (no such creature types
-        exist in the data model).
+        30 ft); dice = (level + 1) // 2. Only TOTAL concealment (miss chance
+        >= 50%) denies the sneak attack; partial concealment (blurred 20%)
+        does not (CRB rogue "Sneak Attack": "cannot sneak attack while
+        striking a creature with total concealment"). PF1e (CRB): undead and
+        constructs are immune to precision damage — ooze/plant immunity is not
+        modeled (no such creature types exist in the data model).
         """
-        if concealment > 0 or not self._has_feat(attacker, _SNEAK_ATTACK_FEATURE):
+        if (
+            concealment >= _TOTAL_CONCEALMENT_PCT
+            or not self._has_feat(attacker, _SNEAK_ATTACK_FEATURE)
+        ):
             return 0
         if target.combatant.creature_type.lower() in _PRECISION_IMMUNE_TYPES:
             return 0