[MPI] Handle boundary patterns at inter-process block edges

[hubertwojcik]

Goal

Extend the MPI implementation to detect patterns that span the boundary between blocks owned by adjacent processes.

Tasks

• Each process sends the last L-1 bytes of its block to the next process
• Each process searches the overlap region formed by its tail and the next process's head
• Rank 0 collects and deduplicates boundary matches
• Test with a corpus containing patterns designed to cross process boundaries

Acceptance Criteria

• Full match output matches sequential baseline on all test inputs
• Boundary exchange uses non-blocking MPI_Isend/MPI_Irecv to overlap communication and computation