Add stable delegation API surface

[p0mvn]

Summary

• Add wallet-facing round, precompute, delegate, phases, pir, transport, error, and prelude modules for delegation setup/precompute/proving flows.
• Rename optional client features to pir and tree-sync while preserving legacy aliases during migration.
• Document the new lifecycle API and add focused tests for bundle setup and persisted delegation phase derivation.

Test plan

• cargo check -p zcash_voting --features pir,tree-sync --examples
• cargo test -p zcash_voting --features pir,tree-sync --lib