feat(dummies): add ContainingEntry to pin a dictionary entry's value

[Reefact]

Summary

AnyDictionary.ContainingKey(TKey) forces a key into the generated dictionary, but its value was still drawn arbitrarily from the value generator — so a test could not pin a specific key/value pair. This adds ContainingEntry(TKey, TValue), which pins the value for a required key while leaving the other entries arbitrary.

Closes #288. The method name (ContainingEntry, over Containing(TKey, TValue) or a chained ContainingKey(...).WithValue(...)) was chosen with the maintainer.

Type of change

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

Changes

• Add AnyDictionary.ContainingEntry(TKey key, TValue value): it forces the key in exactly as ContainingKey does (so it inherits the out-of-domain cardinality credit) and records key → value in an immutable, comparer-aware map that Generate() consults before falling back to the value generator. With(...) carries the map across later constraints, keeping the recipe immutable.
• Conflict semantics reuse the existing machinery, no new rule: dictionary keys are distinct, so pinning the same key twice — or an entry and a plain ContainingKey for the same key — fails eagerly with ConflictingAnyConstraintException through the key-distinctness validation already in CollectionState.
• The factory keeps its three-argument constructor (now delegating to a private four-argument one), so Any.DictionaryOf is untouched.
• Declare the new member in both per-TFM PublicAPI.Unshipped.txt baselines (netstandard2.0 and net8.0).
• Add tests: the pinned value surfacing with an out-of-domain key (value proven pinned because it lies outside the value generator's domain), and the duplicate-key conflict (both the two-entry and the entry-plus-ContainingKey forms).

Testing

• dotnet build FirstClassErrors.sln — Build succeeded, 0 Warning(s), 0 Error(s) (both the netstandard2.0 and net8.0 legs of Dummies; the PublicAPI analyzer is clean).
• dotnet test FirstClassErrors.sln — full suite not run locally; ran dotnet test Dummies.UnitTests instead: 453 passed (2 new), 0 failed, 0 skipped.
• Analyzer tests pass (FirstClassErrors.Analyzers.UnitTests) — not run; no analyzer change in this PR.

Documentation

• Public API / error documentation updated — the new member is declared in the per-TFM PublicAPI.*.txt baselines.
• README / doc/ updated
• French translation (doc/handwritten/for-users/README.fr.md) updated if user-facing behavior changed
• No documentation change required

Architecture decisions

• No architectural decision in this pull request — an additive method reusing the existing key-containment / cardinality-credit machinery (ADR-0013) and the distinct-key conflict path unchanged.
• New decision recorded — ADR drafted as Proposed: ADR-____
• Supersedes an existing ADR — successor proposed, status not flipped: ADR-____
• ⚠️ Conflicts with an existing ADR — flagged for the maintainer: ADR-____

Related issues

Closes #288.