Improve crossword puzzle solver implementation

[Ewanjohndennis]

The original is_valid returned False for any non-empty cell, which meant two words sharing a letter could never be placed together. Since crossword grids are built on exactly those intersections, this broke the core use case. Related to that, remove_word during backtracking blindly blanked every cell of the removed word, wiping out letters that belonged to already-placed crossing words. The fix snapshots the grid before placement and only clears cells that were empty beforehand. There was also a mutation bug: words.remove(word) does an O(n) scan and modifies state shared across the call stack, replaced here with a local slice per frame. On top of the fixes, the solver now tries the longest word first at each level, a standard "most-constrained variable" heuristic that cuts down backtracks significantly on larger inputs.

Describe your change:

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Add or change doctests?
• Documentation change?

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