Add frozen combat modelling.

This doesn't do anything yet, but sets up the data model handling for
frozen combat. The next step is to show combat in the map view, since
that will be helpful when debugging the step after that one: resolving
frozen combat.

This would benefit from caching the Shapely data for SAM threat zones.
Right now it's generating them once per tick and the stuttering is
visible at max speed.

https://github.com/dcs-liberation/dcs_liberation/issues/1680

game/sim/combat/joinablecombat.py

@@ -0,0 +1,25 @@
+from __future__ import annotations
+
+from abc import ABC, abstractmethod
+from collections.abc import Iterator
+from typing import TYPE_CHECKING
+
+from .frozencombat import FrozenCombat
+
+if TYPE_CHECKING:
+    from game.ato import Flight
+
+
+class JoinableCombat(FrozenCombat, ABC):
+    def __init__(self, flights: list[Flight]) -> None:
+        self.flights = flights
+
+    @abstractmethod
+    def joinable_by(self, flight: Flight) -> bool:
+        ...
+
+    def join(self, flight: Flight) -> None:
+        self.flights.append(flight)
+
+    def iter_flights(self) -> Iterator[Flight]:
+        yield from self.flights