Design chunking for C3a and C3b share encryption

[0xjei]

Parent: #1736

Scope

Reduce the C3a and C3b share-encryption circuit size. The current size is approximately 10M constraints.

Investigate whether chunking can apply to C3a and C3b. One possible direction is to separate ct0 and ct1, similar to the approach used for Greco. The design requires review with @zara Javar before implementation.

Completion criteria

• Measure the current C3a and C3b constraint sizes.
• Evaluate chunking and the ct0/ct1 split as design options.
• Confirm that the selected design preserves proof binding, aggregation, and verification behavior.
• Review the design with @zara Javar.
• Implement the selected approach or document the technical blocker and next step.

[0xjei]

currently in progress from @auryn-macmillan at #1916.

note that @zahrajavar still needs to understand if these are worth or not.