Result pattern with a closed error hierarchy (C#)

A way to make a method's failure modes part of its return type without OneOf's positional Tn tedium or a flat opaque Error blob. Emulates a discriminated union using a sealed record hierarchy.

What to call it

There is no single name. It is three named things stapled together:

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2. Repo-local docs: AGENTS.md, PROJECT.md, README.md, docs/, adr/
3. Active issue/PR/task description