Add PRD for MintPlayer.AspNetCore.Endpoints library

[PieterjanDeClippel]

Summary

• Adds a Product Requirements Document for a new MintPlayer.AspNetCore.Endpoints library
• Class-per-endpoint mapping with constructor injection, compile-time route enforcement via static abstract members, and source-generated endpoint discovery
• Content-negotiated body parsing using MVC input formatters, route groups via IMemberOf<TGroup>, attribute support ([Authorize], [Tags], etc.) per endpoint and per group

Design highlights

• 3 NuGet packages: Abstractions (interfaces + base classes), Runtime (mapping extension), Generator (source generator)
• Partial class trick: user writes partial class X : IPostEndpoint<T>, generator adds partial class X : PostEndpoint<T>
• Abstract base classes: BodyEndpoint<T> (POST/PUT/PATCH with MVC formatter support) and NonBodyEndpoint<T> (GET/DELETE with forced explicit binding)
• Multi-assembly support with [assembly: EndpointsMethodName("...")] override
• All 11 design questions resolved — ready for implementation

Test plan

• Review PRD for completeness and correctness
• Validate interface and base class hierarchy design
• Confirm source generator approach (Task A: partial classes, Task B: mapping extension)
• Approve for implementation phase

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