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>

src/pf1e_simulator/rng.py

@@ -0,0 +1,77 @@
+"""Seeded and scripted RNG protocol for deterministic Monte Carlo simulation.
+
+Design: every die roll in the engine flows through the `Rng` protocol.
+`SeededRng` wraps `random.Random(seed)` for reproducible streams.
+`ScriptedRng` pops from a fixed queue — used in tests to script exact outcomes.
+
+Security note: `random.Random` is NOT cryptographically secure.
+This is by design — the simulator needs reproducibility, not security.
+"""
+
+from __future__ import annotations
+
+import random
+from collections import deque
+from typing import Protocol
+
+_EXHAUSTED_MSG = "ScriptedRng queue exhausted"
+
+
+class RngExhaustedError(Exception):
+    """Raised when a ScriptedRng queue is exhausted."""
+
+
+class Rng(Protocol):
+    """Protocol for deterministic die-roll sources."""
+
+    def roll_die(self, sides: int) -> int:
+        """Return a uniform integer in 1..=sides."""
+        ...
+
+    def d20(self) -> int:
+        """Return roll_die(20)."""
+        ...
+
+    def choice_index(self, n: int) -> int:
+        """Return a uniform integer in 0..n-1 for deterministic tie-breaks."""
+        ...
+
+
+class SeededRng:
+    """Rng backed by random.Random(seed) for reproducible streams."""
+
+    def __init__(self, seed: int) -> None:
+        self._rng = random.Random(seed)
+
+    def roll_die(self, sides: int) -> int:
+        return self._rng.randint(1, sides)
+
+    def d20(self) -> int:
+        return self.roll_die(20)
+
+    def choice_index(self, n: int) -> int:
+        return self._rng.randrange(n)
+
+
+class ScriptedRng:
+    """Rng that pops predetermined values from a queue.
+
+    Each call to roll_die, d20, or choice_index consumes one value.
+    Raises RngExhaustedError when the queue is empty.
+    """
+
+    def __init__(self, queue: list[int]) -> None:
+        self._queue: deque[int] = deque(queue)
+
+    def roll_die(self, _sides: int) -> int:
+        if not self._queue:
+            raise RngExhaustedError(_EXHAUSTED_MSG)
+        return self._queue.popleft()
+
+    def d20(self) -> int:
+        return self.roll_die(20)
+
+    def choice_index(self, _n: int) -> int:
+        if not self._queue:
+            raise RngExhaustedError(_EXHAUSTED_MSG)
+        return self._queue.popleft()