refactor(zkvm,gkr): dedup ECC proof preparation and simplify main-eval wiring

[hero78119]

Refactor proof wiring to remove duplicated ECC preparation logic and simplify main-eval handling.
This change centralizes shared helper logic in scheme/utils, unifies ECC prover inputs across CPU/GPU, and streamlines proof assembly order.
We also removes n_instance from GKR main-eval wiring so main evaluations consistently use witin + fixed + structural_witin.

change highlight

• dedup: move shared helpers (extract_ecc_quark_witness_inputs, derive_ecc_bridge_claims, split_rotation_evals, assign_group_evals) into common utils
• api cleanup: EccQuarkProver::prove_ec_sum_quark now takes (cs, input) and returns Option<EccQuarkProof> (no ECC path -> None)
• prover flow cleanup: ECC witness extraction/prep removed from call sites; ECC proving triggered via unified API
• ordering update: tower-related checks/wiring happen before ECC bridge usage; verifier side aligns with this order
• gkr simplification: remove n_instance from layer/main-eval accounting and related offsets/assertions
• net effect: less duplicated code, clearer ownership of ECC bridge data, and simpler main witness/eval wiring

summary of proving flow (simplified ascii)

build_main_witness
-> generate_tower_witness
-> prove_tower_relation
-> prove_ecc_quark (optional, if ecc ops exist)
-> prove_rotation (optional)
-> prove_main_constraints (inject rotation/ecc bridge evals when present)
-> open_pcs
-> assemble_chip_proof