Preserve single errors through Combine; mint validation failures as validation errors

[Vulthil]

Summary

• Combine now matches Zip: a single failed result's original error propagates unwrapped; only 2+ failures aggregate into a ValidationError. Previously even one failure was wrapped, turning e.g. a NotFound into a 400-mapped validation error downstream.
• ValidationPipelineBehavior now mints inner validation errors via Error.Validation instead of the unrelated Error.Problem factory (which had zero other src usage and a copy-pasted XML summary, now fixed).
• Documented the Combine/Zip aggregation rule once in docs/articles/result-pattern.md, gave the ErrorType enum real member docs (with HTTP-mapping intent, coordinated with the parallel Vulthil.SharedKernel.Api mapping work), and documented three sharp edges prominently (implicit null → Result<T> failure, reference-only equality, and the Error.None invariant).
• Widened the Result constructor's Error.None invariant check from value- to reference-equality, so a custom error that merely looks like Error.None (same empty code/description) can be used as a real failure error.

Verification

• Re-verified all findings against current main before changing anything.
• New/changed Combine tests confirmed to fail on unfixed main, then pass after the fix.
• Full solution build clean; Vulthil.Results.Tests 197/197, Vulthil.SharedKernel.Application.Tests 26/26.
• PublicAPI.Shipped/Unshipped.txt untouched — no public signature changes.

Test plan

• dotnet build Vulthil.SharedKernel.slnx — 0 warnings, 0 errors
• Vulthil.Results.Tests (net10.0) — 197/197 passed
• Vulthil.SharedKernel.Application.Tests (net10.0) — 26/26 passed
• Confirmed new Combine expectations fail on unfixed main (reverted src/, reran, restored)

Backport to v1.0: no — behavioral semantics changes are minor-appropriate; ships in 1.1.0.

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.