Development policy: serious engine ambition and quality bar

docs/DEVELOPMENT-POLICY.md

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 Parlyn is developed foundation first. The active scope is completed before optional new systems are promoted into implementation.
 
+## Product ambition and quality bar
+
+Parlyn aims to become a serious, modern, high-quality open-source game engine
+for independent developers and small teams, with a distinctive 2.5D-first
+workflow and credible additional 3D support. A limited prototype is an
+intermediate milestone, not the intended final product.
+
+Godot, Unity and Unreal Engine are reference points for workflow quality,
+capability and developer expectations, not promises of feature parity or an
+instruction to copy every subsystem. Parlyn should earn adoption through
+dependable tools, coherent workflows and useful distinctive capabilities.
+
+Small roadmap steps constrain the current implementation scope, not the
+long-term quality ambition. Evaluate completed workflows for usability,
+reliability, data safety, rendering correctness, performance on stated target
+hardware, documentation and practical usefulness. Validate progress with
+automated checks, human Windows acceptance and, as runtime capabilities mature,
+a small representative playable project. Do not substitute feature counts,
+marketing claims or predicted future AI improvements for evidence.
+
+For asset authoring, choose deliberately between native tools, optional plugins
+and dependable bridges to external applications. An external workflow must be
+clear and well tested, not used as an excuse for an incomplete engine workflow.
+A full native painter or mesh sculptor is not currently approved roadmap work;
+terrain shaping, mesh sculpting and PBR texture painting must be assessed as
+distinct capabilities at a future planning review.
+
+This ambition was explicitly reaffirmed by Andreas Holzer on 2026-09-13.
+It does not bypass the scope, review or acceptance rules below.
+
+## Required design review — established engines and Parlyn's advantage
+
+Before implementing each bounded roadmap step, explicitly ask:
+**How do Godot, Unity and Unreal Engine solve this workflow, and can Parlyn
+make it better for its intended developers?**
+
+Compare the relevant behavior, interaction, architecture and limitations using
+current official documentation where available. Record a concise conclusion in
+the step's design notes or pull request: the reference approach, Parlyn's chosen
+approach, any intended improvement and the trade-offs. Do not claim superiority
+without evidence; validate improvements through tests and human acceptance.
+
+Godot's modular, extensible approach is the preferred realistic starting point
+for Parlyn, not a mandatory architecture to copy. Consider optional plugins and
+well-integrated external-tool bridges before adding large specialist subsystems
+to the core. Native tools remain an option where the workflow clearly warrants
+them and an explicit future planning review approves the scope.
+
+“Better” may mean clearer controls, fewer steps, safer persistence, easier setup,
+more accessible defaults or stronger 2.5D/3D consistency; it need not mean more
+features. Use the comparison to improve the active step, not to import unrelated
+features or interrupt the roadmap. The maturity and resources of the reference
+engines are context, not promises of immediate feature parity.
+
+This review rule was requested by Andreas Holzer on 2026-09-13.
+
 ## First stable release rule
 
 The project's primary objective is a first genuinely stable release of the engine. This means a coherent, installable and testable Parlyn baseline rather than a collection of partially implemented ideas.