[CaffeinatedCoder]

Closes #84

Problem

A caught exception was lost on the way to a Fault<TFault>. The MapError that turns it into the fault an endpoint answers dropped it, so a log of the refusal could not say what failed underneath (decision 52).

Change

• Core: Fault<TFault>.Cause, set by Fault.Of(reason, detail, cause) and the constructor of the same shape; Of(reason, detail, cause) on Fault<T1, T2> and Fault<T1, T2, T3>. WithRule and with keep it; a null cause is refused. Equality and the hash code are written by hand over the reason, detail and rule, so the cause takes no part; ToString() prints as 2.2 did.
• AspNetCore: FaultProblem<TFault>.Cause, and IFaultFeature.Cause as a default member, so a host's own implementation still builds. Nothing of the cause is written into the problem.
• Docs: the AspNetCore README's new "What failed underneath" (two compiled samples), the core README's fault section, the changelog, the option-result note; the roadmap item goes.
• Probe corpus: the two C03 seeds that patch FaultProblem.cs and TypeKitHttpResultExtensions.cs are regenerated against the new source. The private Problem() helper stays the composed overloads' alone, so each-error-own-set still breaks composed faults only.

Tests

• FaultTests, ComposedFaultTests: the cause is kept, refused when null, kept by WithRule and with, and left out of equality, hashing and printing.
• FaultResponsePartsTests: over the four Fault<TFault> overloads and a composed one, the cause reaches IFaultFeature.Cause and the problem has exactly the members a fault without a cause writes, naming neither the exception's type nor its message.
• Seeded and caught: equality over the cause (2 tests fail), a dropped cause in ToHttpResult (6), the cause's message written into the problem (5).
• Full suite: 18,228 passed. Package validation against 2.2.0 passes for both packages. prober seeds --probe C03ComposedProbe: 5 of 5 seeds caught.
• Not run here: the seeds of the other probes whose footprint includes the changed files (seeds --all before the tag).

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