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_dice.py

@@ -0,0 +1,196 @@
+"""Tests for dice notation parsing and DiceExpr evaluation."""
+
+from __future__ import annotations
+
+from collections import Counter
+
+import pytest
+
+from pf1e_simulator.dice import DiceExpr, DiceParseError, parse_dice
+from pf1e_simulator.rng import ScriptedRng
+
+# ── Parsing: accepted forms ──────────────────────────────────────────────────
+
+class TestParseDiceAccepted:
+    """Given: valid dice notation strings
+    When: parse_dice is called
+    Then: returns a correct DiceExpr."""
+
+    def test_2d6_plus_3(self) -> None:
+        expr = parse_dice("2d6+3")
+        assert expr.count == 2
+        assert expr.sides == 6
+        assert expr.bonus == 3
+
+    def test_2d6_minus_1(self) -> None:
+        expr = parse_dice("2d6-1")
+        assert expr.count == 2
+        assert expr.sides == 6
+        assert expr.bonus == -1
+
+    def test_1d4_no_bonus(self) -> None:
+        expr = parse_dice("1d4")
+        assert expr.count == 1
+        assert expr.sides == 4
+        assert expr.bonus == 0
+
+    def test_d20_shorthand(self) -> None:
+        expr = parse_dice("d20")
+        assert expr.count == 1
+        assert expr.sides == 20
+        assert expr.bonus == 0
+
+    def test_flat_integer(self) -> None:
+        expr = parse_dice("7")
+        assert expr.count == 0
+        assert expr.sides == 0
+        assert expr.bonus == 7
+
+    def test_flat_negative(self) -> None:
+        expr = parse_dice("-5")
+        assert expr.count == 0
+        assert expr.sides == 0
+        assert expr.bonus == -5
+
+    def test_flat_zero(self) -> None:
+        expr = parse_dice("0")
+        assert expr.count == 0
+        assert expr.sides == 0
+        assert expr.bonus == 0
+
+    def test_large_dice(self) -> None:
+        expr = parse_dice("10d100+50")
+        assert expr.count == 10
+        assert expr.sides == 100
+        assert expr.bonus == 50
+
+    def test_whitespace_stripped(self) -> None:
+        expr = parse_dice("  2d6+3  ")
+        assert expr.count == 2
+        assert expr.sides == 6
+        assert expr.bonus == 3
+
+
+# ── Parsing: rejected forms ──────────────────────────────────────────────────
+
+class TestParseDiceRejected:
+    """Given: invalid dice notation strings
+    When: parse_dice is called
+    Then: raises DiceParseError carrying the offending text."""
+
+    @pytest.mark.parametrize(
+        "text",
+        [
+            "",
+            "sizeRoll(1, 8, @size)",
+            "2x6",
+            "1d",
+            "d",
+            "2d0",
+            "0d6",
+            "2d 6",
+            "abc",
+            "d6+",
+        ],
+    )
+    def test_rejects_invalid(self, text: str) -> None:
+        with pytest.raises(DiceParseError) as exc_info:
+            parse_dice(text)
+        assert text in str(exc_info.value)
+
+
+# ── Mean calculation ─────────────────────────────────────────────────────────
+
+class TestDiceExprMean:
+    """Given: a DiceExpr
+    When: mean() is called
+    Then: returns count * (sides + 1) / 2 + bonus."""
+
+    def test_2d6_plus_3_mean(self) -> None:
+        assert DiceExpr(count=2, sides=6, bonus=3).mean() == 10.0
+
+    def test_1d4_mean(self) -> None:
+        assert DiceExpr(count=1, sides=4, bonus=0).mean() == 2.5
+
+    def test_d20_mean(self) -> None:
+        assert DiceExpr(count=1, sides=20, bonus=0).mean() == 10.5
+
+    def test_flat_bonus_mean(self) -> None:
+        assert DiceExpr(count=0, sides=0, bonus=7).mean() == 7.0
+
+    def test_negative_bonus_mean(self) -> None:
+        assert DiceExpr(count=1, sides=6, bonus=-2).mean() == 1.5
+
+
+# ── Roll: uses RNG correctly ────────────────────────────────────────────────
+
+class TestDiceExprRoll:
+    """Given: a DiceExpr and a ScriptedRng
+    When: roll(rng) is called
+    Then: consumes exactly `count` values from the RNG."""
+
+    def test_2d6_plus_3_uses_two_rolls(self) -> None:
+        rng = ScriptedRng([3, 4])
+        expr = DiceExpr(count=2, sides=6, bonus=3)
+        assert expr.roll(rng) == 3 + 4 + 3  # 10
+
+    def test_1d20_uses_one_roll(self) -> None:
+        rng = ScriptedRng([15])
+        expr = DiceExpr(count=1, sides=20, bonus=0)
+        assert expr.roll(rng) == 15
+
+    def test_flat_bonus_uses_no_rolls(self) -> None:
+        rng = ScriptedRng([])
+        expr = DiceExpr(count=0, sides=0, bonus=7)
+        assert expr.roll(rng) == 7
+
+    def test_negative_bonus(self) -> None:
+        rng = ScriptedRng([6, 6])
+        expr = DiceExpr(count=2, sides=6, bonus=-1)
+        assert expr.roll(rng) == 6 + 6 - 1  # 11
+
+
+# ── Distribution: exhaustive 2d6+3 ──────────────────────────────────────────
+
+class TestDiceExprDistribution:
+    """Given: 2d6+3 enumerated over all 36 face pairs
+    When: probabilities are computed
+    Then: they match the theoretical distribution."""
+
+    def test_2d6_plus_3_distribution(self) -> None:
+        totals: Counter[int] = Counter()
+        for d1 in range(1, 7):
+            for d2 in range(1, 7):
+                rng = ScriptedRng([d1, d2])
+                expr = DiceExpr(count=2, sides=6, bonus=3)
+                totals[expr.roll(rng)] += 1
+
+        assert totals[5] == 1   # (1,1) → 1+1+3=5
+        assert totals[7] == 3   # (1,3),(2,2),(3,1) → sum=4+3=7
+        assert totals[10] == 6  # sum=7: (1,6)..(6,1) → 7+3=10
+        assert totals[15] == 1  # (6,6) → 12+3=15
+        assert sum(totals.values()) == 36
+
+
+# ── DiceExpr.flat factory ────────────────────────────────────────────────────
+
+class TestDiceExprFlat:
+    """Given: DiceExpr.flat(n)
+    When: called with any integer
+    Then: returns count=0, sides=0, bonus=n."""
+
+    def test_flat_positive(self) -> None:
+        expr = DiceExpr.flat(10)
+        assert expr.count == 0
+        assert expr.sides == 0
+        assert expr.bonus == 10
+        assert expr.mean() == 10.0
+
+    def test_flat_negative(self) -> None:
+        expr = DiceExpr.flat(-3)
+        assert expr.bonus == -3
+
+    def test_flat_roll_returns_bonus(self) -> None:
+        rng = ScriptedRng([])
+        expr = DiceExpr.flat(5)
+        assert expr.roll(rng) == 5