perf: Factor Eq in ZeroCheck when possible

[Tabaie]

When a Zero-Check has only one "evaluation point", at every round a prefix of the eq term (containing an indeterminate) can be factored out and computed by the verifier. This reduces the number of Fiat-Shamir hashes benefitting the verifier directly (at the cost of a trivial amount of extra work to compute the eq terms at the end of a sum-check run). For the prover, 1/d+1 of the work each round will be saved.

BenchmarkGkrPoseidon2 was benchmarked on an hpc6a.48xlarge machine with the following results:

  ┌─────────────┬───────────────┬────────────────────┬──────────────────┬─────────────────┐
  │   Metric    │    master     │ perf/gkr/factor-eq │      Delta       │   Improvement   │
  ├─────────────┼───────────────┼────────────────────┼──────────────────┼─────────────────┤
  │ ns/op       │ 1,791,523,877 │ 1,522,833,929      │ -15.0%           │ 1.18x speedup   │
  ├─────────────┼───────────────┼────────────────────┼──────────────────┼─────────────────┤
  │ Constraints │ 3,547,650     │ 3,324,843          │ -6.3% (-222,807) │ 1.07x reduction │
  ├─────────────┼───────────────┼────────────────────┼──────────────────┼─────────────────┤
  │ B/op        │ 736,144,887   │ 724,871,928        │ -1.5% (-11.3 MB) │ 1.02x reduction │
  ├─────────────┼───────────────┼────────────────────┼──────────────────┼─────────────────┤
  │ allocs/op   │ 298,196       │ 281,781            │ -5.5% (-16,415)  │ 1.06x reduction │
  └─────────────┴───────────────┴────────────────────┴──────────────────┴─────────────────┘

---

Note

High Risk
Touches core GKR proving/verification and transcript-binding behavior; any mistake could break proof soundness or cause verifier/prover divergence despite being a performance-focused change.

Overview
Introduces constraint.GkrSingleSourceZeroCheckLevel and wires it into schedule serialization (marshal.go) and level dispatch in generated and curve-specific GKR backends.

Implements a new single-source zerocheck sumcheck variant that factors out the eq polynomial (precomputing suffix eq tables and adjusting prover/verifier round polynomials), while refactoring shared zerocheck setup into zeroCheckBase and consolidating verifier setup logic.

Also changes gateEvaluatorPool from a map-backed set to a slice-based stack for cheaper reuse, and moves the bulky zerocheck/sumcheck claim implementations out of gkr.go into sumcheck.go templates/generated code.

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