feat: add example for printing constraints

[hawkadrian]

Adds example showing how to print constraints from compiled constraint system using type assertion and GetR1Cs()/GetSparseR1Cs() methods.

Fixes #1640

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Note

Adds concise examples under examples/print_constraints to print R1CS and SparseR1CS constraints from compiled circuits using type assertions.

• Examples:
  • New package examples/print_constraints:
    • doc.go: package documentation for printing constraints from compiled systems.
    • print_constraints_r1cs_test.go: adds CubicCircuit and Example_printR1CS demonstrating compilation with r1cs.NewBuilder, type-asserting GetR1Cs(), and printing system stats and each R1C.
    • print_constraints_scs_test.go: adds Example_printSparseR1CS demonstrating compilation with scs.NewBuilder (PLONK), type-asserting GetSparseR1Cs(), and printing system stats and each SparseR1C.

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[Copilot]

Pull request overview

This PR adds comprehensive examples demonstrating how to print constraints from compiled constraint systems, addressing issue #1640. The examples show developers how to introspect and debug their circuits by accessing and printing the underlying constraints.

Key Changes:

• New print_constraints package with examples for both R1CS and SparseR1CS (PLONK) constraint systems
• Demonstrates type assertion patterns to access constraint-specific methods (GetR1Cs() and GetSparseR1Cs())
• Includes detailed documentation and example outputs showing constraint structure

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
examples/print_constraints/doc.go	Package documentation explaining the purpose and usage of constraint printing examples
examples/print_constraints/print_constraints_r1cs_test.go	Defines CubicCircuit and demonstrates printing R1CS constraints with example output
examples/print_constraints/print_constraints_scs_test.go	Demonstrates printing SparseR1CS (PLONK) constraints using the shared CubicCircuit definition

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