Dummies: add a top-level choice combinator Any.OneOf<T> / Any.ElementOf<T>

[Reefact]

Problem

Picking an arbitrary element from a caller-supplied set is one of the most common dummy needs in real suites ("any of the three configured currencies", "one of the states in this table"). Today OneOf exists only inside typed builders (AnyInt32.OneOf, AnyEnum.OneOf, …) — there is no top-level way to draw from a set of domain objects or arbitrary values. Every user hand-rolls the same code, and typically forgets the seeded source, silently breaking Reproducibly for that draw — the exact trap the library exists to prevent:

// Today — hand-rolled, and not seed-aware:
var currencies = new[] { eur, usd, gbp };
var currency   = currencies[new Random().Next(currencies.Length)];   // ambient seed ignored!

Impact

A high-frequency, philosophy-consistent need is unserved by a library whose whole point is expressive, reproducible test data. This is the highest-leverage addition in the audit's gap analysis (a Must-Have): every consumer, most weeks.

Direction

Add top-level factories that draw from a fixed pool over the ambient source (mirrored on AnyContext):

// Proposed — seed-aware, philosophy-consistent, eagerly validated (empty pool throws):
Currency currency = Any.OneOf(eur, usd, gbp).Generate();
Order    order    = Any.ElementOf(existingOrders).Generate();

• Any.OneOf(params T[]) and Any.ElementOf(IReadOnlyList) (an IEnumerable overload materializes once).
• Constructive: a single uniform draw from the pool; no filtering.
• Implement ICardinalityHint (distinct count of the pool) so it composes with distinct collections for free.
• Empty pool ⇒ eager ArgumentException (a null element in the pool is allowed — it is a valid arbitrary choice).

Acceptance criteria

• Any.OneOf(a, b, c).Generate() and Any.ElementOf(list).Generate() return a member of the pool, drawing from the ambient/context source (reproducible under Reproducibly/WithSeed).
• An empty pool throws eagerly with a clear message.
• The generator advertises cardinality so SetOf(Any.OneOf(...)).WithCount(n) gates correctly.
• Mirrored on AnyContext; covered by the parity guards.
• Documented in the Dummies user guide with an example.

Context

Surfaced by the 2026-07-20 architecture & design audit of the standalone Dummies library (doc/handwritten/for-maintainers/audit/2026-07-20-dummies-architecture-and-design-audit.md, §10 Must-Have; branch claude/dummies-architecture-audit-cru5aq).

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Filed from the Claude Code session that produced the 2026-07-20 Dummies architecture & design audit: https://claude.ai/code/session_01GvrWFVoG74xiys3oyEft1G