Add optimal graph coloring to minimize shared FIFO buffer allocations

[potatoeeh]

The CMSIS-Stream scheduler uses nx.coloring.greedy_color() to assign shared buffers to FIFOs via graph coloring. This greedy heuristic can use more colors (buffers) than the graph's chromatic number.

This adds an exact backtracking-based coloring algorithm (optimal_coloring) that finds the minimum number of colors. Because the solver is expensive on large graphs, it includes a configurable visit budget (max_visits, default 131072). If the budget is exceeded, the function returns None, allowing the caller to fall back to the existing greedy algorithm.

Two-tier coloring strategy:

1. Try exact backtracking (optimal, but potentially expensive)
2. If budget exceeded, fall back to greedy (suboptimal, but fast)

Also adds Tests/test_optimal_coloring.py exercising correctness, greedy-vs-optimal gap, and budget fallback behavior.