[OPENMP] Implement text chunking and parallel search with OpenMP

[hubertwojcik]

Goal

Parallelize the Aho-Corasick search phase using OpenMP by dividing the input text into equal chunks processed concurrently by multiple threads.

Tasks

• Divide text into N chunks (N = omp_get_max_threads())
• Each thread runs the sequential search on its assigned chunk
• Merge per-thread match lists into a single result, adjusting offsets
• Add CLI flag --threads N to control thread count
• Verify correctness: results must match sequential baseline on all test inputs

Acceptance Criteria

• Match output identical to sequential baseline (excluding boundary matches — handled in next issue)
• Speedup measured and logged for 2, 4, 8, 10-core configurations on M4 Pro
• No data races detected under ThreadSanitizer