plonk(recursion): handle zero public inputs in PI computation and add tests

[Forostovec]

This change fixes a crash in the recursive PLONK verifier when a circuit has zero public inputs. Previously, PrepareVerification unconditionally accessed witness.Public[0], causing an out-of-range panic. We now initialize PI to zero and only sum Lagrange contributions when len(witness.Public) > 0, matching the native backend’s behavior for zero-public-input circuits. A dedicated test was added to ensure recursive verification succeeds with an inner circuit that has no public inputs.

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Note

Safely compute PI when there are zero public inputs and add a test verifying recursive proof works with an inner circuit that has no public inputs.

• Verifier (std/recursion/plonk/verifier.go):
  • Initialize pi to 0 and compute Lagrange sum only when len(witness.Public) > 0.
  • Restructure PI loop: guard first term, adjust wPowI/lagrange updates to avoid out-of-range access.
• Tests (std/recursion/plonk/verifier_test.go):
  • Add InnerCircuitZeroPublic and helper to produce proof with zero public inputs.
  • Add TestZeroPublicInputsBW6InBN254 to ensure recursive verification succeeds with empty witness.Public.

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