PATH-01: Shortest path solver (BFS)

Implement BFS from entry to exit, returning the path as a sequence of N/E/S/W letters.

Acceptance criteria: returned path is valid (respects walls) and shortest for both PERFECT modes.

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**Day:** Day 6  
**Milestone:** Days 4-6 - Modes & Output  
**Estimate:** M  
**Depends on:** GEN-07, GEN-08