refactor(dummies): name the replay guidance and its snippet distinctly

[Reefact]

Summary

ReplayHint and ReplayInstruction name a whole and its part — one sentence embedding the code a reader copies — but neither name advertised the containment, nor which was which. The weight was inverted too: the whole was a "hint" (which undersells an exact, verbatim sentence) while the part carried the stronger "instruction". Renamed to ReplayGuidance and ReplaySnippet, so prose contains code reads without a comment. No behavioural change: every generated message is byte-identical.

Type of change

• Refactoring
• Breaking change — public parameter renamed; see note below on why this costs nothing today
• Documentation

Changes

• ReplayHint → ReplayGuidance, PartialReplayHint → PartialReplayGuidance (internal, on RandomSource and both implementations).
• ReplayInstruction → ReplaySnippet (the private helper and the ambient-state property).
• Any.UseSeed(int, string replayInstruction) → replaySnippet, with the PublicAPI baselines updated on both TFMs.
• The same rename in Dummies.Xunit's private helper and in both test suites.
• XML docs now state the whole/part relation outright instead of leaving it to be inferred, and Any.UseSeed(int, string) gains a worked example showing the snippet and the sentence it lands in.
• ADR-0038 amended in place, EN and FR.
• User docs and the two package readmes follow the same vocabulary.

Why the public rename is free

Dummies has never shipped: both PublicAPI.Shipped.txt files are empty, there is no dum-v* tag, and no PackageValidationBaselineVersion is set. No consumer can be affected, which is exactly the pre-1.0 latitude ADR-0011 and ADR-0038 rely on. The box above is ticked because the signature changes, not because anyone is broken.

On amending an accepted ADR

ADR-0038 is Accepted, and the repository's rule is that an accepted ADR is not edited in place. This edit was authorised by @Reefact in the session on the grounds that the terminology is not the decision. Its Decision, Rationale, Alternatives and Consequences are unchanged in substance — only the two words naming the concepts move, including one section heading (Phrase the ambient hint neutrally → Phrase the ambient guidance neutrally). If you would rather this were a superseding ADR than an in-place amendment, say so and I will restructure it.

French

The French docs keep « indication de rejeu » for the sentence — the right French word, where a calque like « guidage » would read as a guidance system — and move the fragment to « extrait de rejeu », the usual rendering of snippet.

Testing

• dotnet build FirstClassErrors.sln — succeeds, no warnings
• dotnet test FirstClassErrors.sln — 11 suites, 1559 tests, 0 failures
• Analyzer tests pass (FirstClassErrors.Analyzers.UnitTests) — 132 passed, as part of the solution run

That the behaviour is unchanged is not merely asserted: the pre-existing tests pin the exact message text ("The arbitrary values were seeded with 1234", "Any.Reproducibly(1234") and pass untouched. A repository-wide grep confirms no occurrence of ReplayHint, ReplayInstruction, replayInstruction, "replay instruction" or « instruction de rejeu » remains.

Not run here: the net472 leg, as before — there is no Mono on the Linux legs, so the framework-floor job proves it.

Documentation

• Public API / error documentation updated
• README / doc/ updated
• French translation updated if user-facing behavior changed

Architecture decisions

• No architectural decision in this pull request

ADR-0038 is touched, but for vocabulary only — no decision is created, superseded or contradicted.

Related issues

Refs #226 — follow-up to #300, which introduced the two concepts.

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