⚡ Bolt: Vectorize lattice fourier transform

[makskliczkowski]

💡 What: Refactored the fourier_transform_lattice function in physics/spectral/greens_function.py to eliminate a nested Python loop. Replaced O(N^2) element-wise phase updates with vectorized matrix operations.
🎯 Why: The original implementation manually iterated over all pairs of spatial coordinates r_i and r_j, accumulating the product greens_function[i, j] * exp(1j * k \cdot (r_i - r_j)). In Python, this type of O(N^2) iteration is very slow. The same calculation can be expressed as a quadratic form $P^T G P^*$ where the vector $P_i = \exp(i k \cdot r_i)$, moving the workload entirely into optimized C/BLAS routines via NumPy.
📊 Impact: Reduces execution time for $N=200$ from ~130ms to ~0.5ms (a roughly 250x speedup), avoiding heavy Python loop overhead. Memory allocation is slightly improved by doing fewer per-element allocations.
🔬 Measurement: Verified using a local performance test script (time measured for single invocations against $N=200$). Verified correctness by running the full project test suite via python3 -m pytest.

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PR created automatically by Jules for task 14517390459703919699 started by @makskliczkowski

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