refactor(justdummies): rename the Dummies library to JustDummies

[Reefact]

Summary

Rename the standalone arbitrary-test-value library from Dummies to JustDummies across the whole repository — projects, folders, public namespace, NuGet package ids, tooling, CI, commit scope, and documentation (EN + FR). The common-noun pattern term dummy/dummies (the generated values) is preserved everywhere it names the concept rather than the library.

Type of change

• Bug fix
• New feature
• Breaking change
• Refactoring
• Analyzer / diagnostic change
• Tests
• Documentation
• Build / CI / tooling

Breaking in principle (public namespace + package ids change), but the library has never been published, so there is no downstream consumer to break.

Changes

• Projects & folders: Dummies* → JustDummies* (JustDummies, JustDummies.UnitTests, JustDummies.Xunit, JustDummies.Xunit.UnitTests), the five .csproj files, and the solution (project GUIDs preserved).
• Public namespace Dummies → JustDummies (+ .Xunit, test namespaces), all using/cref references, and the PublicAPI/** baselines.
• NuGet package ids: JustDummies, JustDummies.Xunit.
• Packaged-asset guard tool: tools/dummies-check → tools/justdummies-check (JustDummiesCheck.csproj, JustDummiesCheckVersion, JUSTDUMMIESCHECK_VERSION).
• CI workflow: .github/workflows/dummies.yml → justdummies.yml (file, workflow name:, job name, concurrency group).
• Commit scope: dummies → justdummies (commit-lint, tools/trains.sh, CONTRIBUTING.md EN + FR, CLAUDE.md, tidy-history command, dependabot prompt/workflow, release-doc pages).
• Docs: EN + FR user/maintainer docs, audits, and ADR contents updated terminologically.
• Showcase site: PackageProjectUrl → https://justdummies.io on both packages (link also added to README.nuget.md); RepositoryUrl stays on the GitHub repository.
• Preserved: the pattern noun dummy/dummies for the generated values, the NuGet dummies tag, and the local dummies variable in the check tool.

Testing

• dotnet build FirstClassErrors.sln — 0 warnings, 0 errors
• dotnet test FirstClassErrors.sln — all suites pass (incl. JustDummies.UnitTests 535, JustDummies.Xunit.UnitTests 14, and the standalone-boundary architecture test)
• Analyzer tests pass (FirstClassErrors.Analyzers.UnitTests — 132)

Documentation

• Public API / error documentation updated (PublicAPI baselines renamed)
• README / doc/ updated
• French translation updated (FR maintainer/user docs kept in sync with the rename)
• No documentation change required

Architecture decisions

• No architectural decision in this pull request

Per the maintainer's explicit direction, the existing ADRs (including ADR-0011 — host the library as a standalone package) were updated in place, terminologically only: the recorded decisions are unchanged, so no ADR is superseded and no status is flipped. Accepted-ADR filename slugs were intentionally left as-is to avoid breaking cross-references.

⚠️ Maintainer follow-ups (deliberate scope choices — say the word to change any)

1. Required status checks — renaming the CI workflow changes the check context from dummies / Dummies packaged-asset compatibility to justdummies / JustDummies packaged-asset compatibility. If the old name is a required check in branch protection, update it in the repo settings; otherwise this PR may show a required check as missing.
2. Release train dum / tag dum-v* kept as-is (terse internal code, no published tags, library moving to its own repo soon). Only its scope/path/label were updated.
3. ADR/audit filename slugs (0011-host-dummies-…, 2026-07-20-dummies-…, etc.) kept; only their contents were updated.

Related issues

None.

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