[PlonK] Introduce unified PlonKPipeline and PlonKKeys facade matching Groth16Pipeline

[satran004]

Background & Objective

ADR-0036 introduced Groth16Keys and Groth16Pipeline, which unified compilation, trusted setup, witness mapping, key storage, and proving into a clean, single-point-of-entry API.

PlonK currently has fragmented setup and proving entry points across tests and modules.

Scope & Tasks

• Implement PlonKKeys AutoCloseable handle for PlonK setup / proving key material.
• Implement PlonKPipeline orchestration owning compilation, .ptau / .zkey loading, cache validation, witness generation, and proving.
• Add unit tests and update examples to demonstrate the unified PlonK pipeline.

Risk Classification

• R1: API consistency and developer ergonomics.
• Reference: docs/zeroj-comprehensive-review-and-readiness-gemini.md (Section 5.3).