perf: early stopping power iteration

[ofabiodev]

Current power iteration always runs the maximum number of iterations, causing unnecessary computation for most regex patterns. This PR will:

• Add early stopping when eigenvector converges
• Preserve spectral radius accuracy
• Improve performance for large NFAs

📊 Performance Gains

Matrix Size	Original Avg (ms)	New Avg (ms)	Improvement
50x50	1.499	0.422	~70%
200x200	10.689	0.999	~90%
500x500	59.448	3.807	~93%