Crossword Constraint Solver

Question

Given a crossword grid, a set of across/down clue entries, and candidate words for each clue, fill the grid so every entry uses one candidate and all crossing letters agree.

## Requirements

- Define data structures for grid cells, clue slots, candidate domains, and crossing constraints.
- Return one valid completed grid or report that none exists.
- Use constraint propagation and backtracking rather than enumerating every combination.
- Choose a search order such as minimum remaining values, and explain tie-breaking.
- Restore domains correctly after a failed branch.
- Handle prefilled letters, duplicate candidate words, disconnected regions, and ambiguous puzzles.

You may propose an interface such as:

```python
def solve_crossword(grid: list[str], entries: list[dict]) -> list[str] | None: ...
```

State the exact entry schema before coding.

## Follow-ups

1. Return all solutions or prove uniqueness.
2. Index a large dictionary by length and letter position.
3. Rank semantically plausible candidates derived from clue text.
4. Parallelize search without duplicating too much work.