[WIP] Upgrade Renderflow to universal transformation engine with DAG

[Copilot]

Thanks for asking me to work on this. I will get started on it and keep this PR's description up to date as I form a plan and make progress.

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<issue_title>🧠 Renderflow v2 — Universal Transformation Engine (DAG + Optimization + Plugins)</issue_title>
<issue_description>### Title: 🧠 Renderflow v2 — Universal Transformation Engine (DAG + Optimization + Plugins)

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Description

Upgrade Renderflow into a universal transformation engine powered by a DAG-based planning system.

This system will support:

• graph-based transformation pipelines
• multi-target optimization (cost + quality)
• reusable intermediate artifacts
• plugin-based extensibility
• AI-powered transformations

This replaces hardcoded pipelines with a declarative, extensible execution model.

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Goals

• transform Renderflow into a universal transformation platform
• support arbitrary input → output pipelines
• optimize execution using cost + quality scoring
• enable reuse across domains (documents, media, AI pipelines)
• eliminate redundant work via DAG merging and caching

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Scope

This effort introduces a new architecture layer including:

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🧱 Core Graph Engine

• DAG-based transformation model
• format nodes + transform edges
• graph traversal and resolution

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⚙️ Optimization & Planning

• cost-based pathfinding
• quality-aware scoring
• multi-objective optimization
• Pareto frontier support

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🔌 Transform System

• dynamic transform registry
• command-based transforms
• function-based transforms
• plugin-based transforms

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🤖 AI Integration

• AI-based transforms (e.g. image → screenplay)
• model tracking and metadata
• caching for expensive operations

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🔁 Multi-Input Nodes

• support for collection-based transforms
• aggregation pipelines (e.g. pages → book)

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⚡ Execution Engine

• DAG execution with topological sorting
• parallel execution of independent nodes
• incremental builds with caching

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📦 Artifact & Cache System

• artifact hashing and identity
• lineage tracking (input → output)
• persistent caching across runs

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📂 Format System

• canonical format definitions
• format detection
• extensible format registry

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🧠 CLI & UX

• graph-based execution (--target, --all)
• optimization modes (speed, quality, balanced, pareto)
• interactive pipeline selection
• graph/debug visualization

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📊 Observability

• structured logging for execution
• DAG visualization
• execution timing metrics

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🔌 Plugin System

• external transform plugins
• WASM or dynamic plugin support
• plugin discovery and validation

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📚 Configuration

• declarative pipeline config (renderflow.yaml)
• transform definitions in config
• validation and schema support

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🚀 Distribution

• integration with existing CLI
• backward compatibility with current behavior
• packaging and distribution support

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Requirements

• maintain compatibility with current renderflow features
• ensure deterministic execution where applicable
• support both local and plugin-based transforms
• keep system modular and extensible

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Acceptance Criteria

• DAG-based transformation system implemented
• pathfinding and optimization working
• execution engine runs graph correctly
• transform registry supports dynamic definitions
• plugin system supports external transforms
• CLI supports graph-based execution
• caching eliminates redundant work
• system is usable across multiple domains
• documentation updated for new architecture</issue_description>

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