feat(dice): seeded RNG protocol and dice notation parser

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

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

tests/test_rng.py

@@ -0,0 +1,115 @@
+"""Tests for the RNG protocol, SeededRng, and ScriptedRng."""
+
+import pytest
+
+from pf1e_simulator.rng import RngExhaustedError, ScriptedRng, SeededRng
+
+
+class TestSeededRngRollDie:
+    """Given: a SeededRng with a fixed seed
+    When: roll_die is called
+    Then: returns values in 1..=sides deterministically."""
+
+    def test_roll_die_returns_in_range(self) -> None:
+        rng = SeededRng(42)
+        for _ in range(100):
+            result = rng.roll_die(6)
+            assert 1 <= result <= 6
+
+    def test_roll_die_d20_in_range(self) -> None:
+        rng = SeededRng(42)
+        for _ in range(100):
+            result = rng.d20()
+            assert 1 <= result <= 20
+
+    def test_d20_is_roll_die_20(self) -> None:
+        rng1 = SeededRng(123)
+        rng2 = SeededRng(123)
+        assert rng1.d20() == rng2.roll_die(20)
+
+
+class TestSeededRngChoiceIndex:
+    """Given: a SeededRng
+    When: choice_index(n) is called
+    Then: returns values in 0..n-1."""
+
+    def test_choice_index_in_range(self) -> None:
+        rng = SeededRng(42)
+        for _ in range(100):
+            result = rng.choice_index(5)
+            assert 0 <= result <= 4
+
+
+class TestSeededRngReproducibility:
+    """Given: two SeededRng instances with the same seed
+    When: each produces 1000 rolls
+    Then: the streams are identical."""
+
+    def test_identical_d20_streams(self) -> None:
+        rng1 = SeededRng(42)
+        rng2 = SeededRng(42)
+        rolls1 = [rng1.d20() for _ in range(1000)]
+        rolls2 = [rng2.d20() for _ in range(1000)]
+        assert rolls1 == rolls2
+
+    def test_identical_choice_index_streams(self) -> None:
+        rng1 = SeededRng(42)
+        rng2 = SeededRng(42)
+        idx1 = [rng1.choice_index(8) for _ in range(1000)]
+        idx2 = [rng2.choice_index(8) for _ in range(1000)]
+        assert idx1 == idx2
+
+    def test_different_seeds_produce_different_streams(self) -> None:
+        rng1 = SeededRng(1)
+        rng2 = SeededRng(2)
+        rolls1 = [rng1.d20() for _ in range(100)]
+        rolls2 = [rng2.d20() for _ in range(100)]
+        assert rolls1 != rolls2
+
+
+class TestScriptedRng:
+    """Given: a ScriptedRng with a queue of ints
+    When: roll_die / choice_index is called
+    Then: returns queued values in order."""
+
+    def test_roll_die_returns_queued_values(self) -> None:
+        rng = ScriptedRng([3, 1, 4, 1, 5])
+        assert rng.roll_die(6) == 3
+        assert rng.roll_die(6) == 1
+        assert rng.roll_die(6) == 4
+
+    def test_d20_returns_queued_values(self) -> None:
+        rng = ScriptedRng([20, 1, 13])
+        assert rng.d20() == 20
+        assert rng.d20() == 1
+        assert rng.d20() == 13
+
+    def test_choice_index_returns_queued_values(self) -> None:
+        rng = ScriptedRng([0, 2, 1])
+        assert rng.choice_index(5) == 0
+        assert rng.choice_index(5) == 2
+        assert rng.choice_index(5) == 1
+
+    def test_raises_on_exhaustion_roll_die(self) -> None:
+        rng = ScriptedRng([5])
+        rng.roll_die(6)
+        with pytest.raises(RngExhaustedError):
+            rng.roll_die(6)
+
+    def test_raises_on_exhaustion_d20(self) -> None:
+        rng = ScriptedRng([10])
+        rng.d20()
+        with pytest.raises(RngExhaustedError):
+            rng.d20()
+
+    def test_raises_on_exhaustion_choice_index(self) -> None:
+        rng = ScriptedRng([0])
+        rng.choice_index(3)
+        with pytest.raises(RngExhaustedError):
+            rng.choice_index(3)
+
+    def test_exhaustion_error_carries_message(self) -> None:
+        rng = ScriptedRng([1])
+        rng.roll_die(6)
+        with pytest.raises(RngExhaustedError, match="exhausted"):
+            rng.roll_die(6)