Replace int[] dictionary keys with a lower-allocation representation

[hutchpd]

Problem

The multi-qubit state representation appears to use int[] keys with structural comparison. This is flexible, but repeated cloning/grouping/copying of arrays may create allocation pressure during gate operations.

Why this matters

For small systems this is probably fine, but multi-qubit operations can grow quickly. Reducing allocations could improve performance and make the library scale better.

Suggested approach

Investigate alternatives such as:

• bit-packed integer/long keys for small qubit counts
• a readonly struct key
• pooled arrays
• immutable value-based basis state representation

This should probably start with measurement rather than immediate refactoring.

Acceptance criteria

• Add a benchmark or small performance test around repeated multi-qubit gate application.
• Compare current int[] key behaviour against at least one alternative representation.
• Document findings before changing the core representation.