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>

tests/test_accuracy_rules.py

@@ -138,7 +138,8 @@ def test_total_concealment_triggers_at_50() -> None:
     assert t.hp == 100
 
 
-def test_concealment_denies_sneak_attack() -> None:
+def test_total_concealment_denies_sneak_attack() -> None:
+    """(A7) total concealment (displaced 50%) denies the sneak attack."""
     sneak = _combatant("atk", level=3, features=("Sneak Attack",))
     eng = _engine(
         [sneak, _combatant("tgt")],
@@ -148,13 +149,37 @@ def test_concealment_denies_sneak_attack() -> None:
     )
     a, t = eng._states
     _add_condition(t, "flat_footed")
-    _add_condition(t, "blurred")
+    _add_condition(t, "displaced")
     result = eng.resolve_attack(a, t, a.combatant.attacks[0])
+    # d20=15 hits, d100=90 > 50% lands; sneak denied by total concealment,
+    # so only the 1d6 weapon die (3) lands.
     assert result.damage == 3
     assert t.hp == 97
     assert not any("sneak" in line for line in eng._transcript)
 
 
+def test_partial_concealment_still_allows_sneak_attack() -> None:
+    """(A7) a blurred (20%) target CAN be sneak attacked — CRB rogue text is
+    "cannot sneak attack while striking a creature with total concealment",
+    so partial concealment no longer denies the precision damage."""
+    sneak = _combatant("atk", level=3, features=("Sneak Attack",))
+    eng = _engine(
+        [sneak, _combatant("tgt")],
+        [(0, 0), (0, 1)],
+        ["players", "monsters"],
+        [15, 90, 6, 6, 4],
+    )
+    a, t = eng._states
+    _add_condition(t, "flat_footed")
+    _add_condition(t, "blurred")
+    result = eng.resolve_attack(a, t, a.combatant.attacks[0])
+    # d20=15 hits, d100=90 > 20% lands; 2d6 sneak (6+6=12) + 1d6 weapon (4)
+    # = 16 precision-inclusive damage.
+    assert result.damage == 16
+    assert t.hp == 84
+    assert any("sneak attack" in line and "2d6" in line for line in eng._transcript)
+
+
 def test_concealment_miss_does_not_consume_deed() -> None:
     eng = _engine(
         [