Grid::removeLocation leaves entities in the removed location pointing at a nonexistent location id

[dmccoystephenson]

Grid::removeLocation (src/grid.cpp) erases the matching Location from the grid's locations vector without touching the entities that location holds:

void Grid::removeLocation(Location& location) {
    std::string targetId = location.getId();
    for (auto i = locations.begin(); i != locations.end();) {
        if (i->getId() == targetId) {
            i = locations.erase(i);
        } else {
            i++;
        }
    }
}

Any Entity* held by the erased Location keeps a locationId (set by Location::addEntity, see src/location.cpp) naming a location that is no longer part of the grid. The entity's gridId is likewise left pointing at the grid. Consequences observable through the public surface:

• Grid::getNumEntities() silently drops those entities from its total, since it sums over surviving locations only.
• Grid::isEntityPresent(entity) returns false while entity.getLocationId() still returns the erased location's id, so the two accessors disagree.
• Grid::getLocation(entity.getLocationId()) throws std::runtime_error("Location not found") for an entity that reports itself as located.

By contrast, Grid::removeEntity calls entity.resetLocationId() and sets the grid id to -1, so the intended convention appears to be that an entity leaving the grid has its ids cleared.

Suggested resolution: decide and document the intended semantics — either removeLocation should call resetLocationId() (and clear gridId) on each entity in the location before erasing it, or the method should refuse to remove a non-empty location. Whichever is chosen, a test asserting the resulting entity state should accompany it.

This gap was noticed while adding direct coverage for Grid::removeLocation in PR #35; it is deliberately left out of that PR, which is scoped to the iterator-invalidation fix tracked by #31.

This issue body was drafted during a Gardener session (https://github.com/Stephenson-Software/gardener).

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drafted by Claude on behalf of Daniel Stephenson