Add Dask Execution for ConformationDAG

[harryswift01]

Goal

Add Dask execution for the conformational map-reduce workflow once chunked serial execution is correct.

Background

The previous frame execution refactor introduced deterministic chunked Dask execution for covariance and neighbour counts. This sub-issue should apply the same principles to conformational analysis, but through the dedicated ConformationDAG.

Scope

• Add a Dask execution path for Pass 1 dihedral angle/histogram collection.
• Add a Dask execution path for Pass 2 conformational state assignment.
• Keep serial chunked execution as the fallback.
• Submit bounded frame-chunk tasks.
• Scatter worker-shared data once where safe.
• Ensure workers return compact partial results.
• Ensure workers do not mutate parent accumulators.
• Reduce completed chunk results deterministically in chunk-index order.
• Release futures and scattered worker data correctly.
• Cancel active futures on failure.
• Keep public configuration unchanged unless a user-facing option becomes clearly necessary.