Glyph to word indexing

[boisvert42]

1. The Bottleneck: O(N) Option Scanning
   In the original establish_arc_consistency, whenever a glyph became "depleted" (count reached 0) in a cell, the algorithm would:
2. Find the crossing slot.
3. Iterate through every single word (other_slot_options) in that crossing slot.
4. Check if that word's letter at the crossing point was the depleted glyph.

If a slot has 10,000 potential words, it performs 10,000 checks every time a letter is eliminated. In a 15x15 grid, this happens thousands of times per second.

2. The Solution: Glyph-to-Word Indexing
   We can introduce a new index, slot_options_by_glyph, inside GridConfig. It is a 4D structure: [slot_id][cell_index][glyph_id] -> Vec.

• Pre-computation: This index is populated once when the GridConfig is created. It maps every possible letter in every cell to the specific subset of words that contain that letter.
• Constant-Time Trigger: In establish_arc_consistency, we now track depleted_glyphs_by_cell. When a glyph count hits zero, we don't scan anything. We simply look up the crossing slot and the crossing cell in our index.
• Targeted Elimination: The index gives us exactly the list of WordIds that are now invalid. We only iterate over the words that must be removed.