Delete Option extensions and tests that nothing else depends on

[KaliCZ]

Follow-up to #36. Strips everything Option-related that isn't actually load-bearing. Option<T> itself stays because Try_Old still exposes Option<TSuccess>/Option<TError> on its public surface and every Coproduct_Old property returns Option<T> — a full migration off Option has to wait until Try and Coproduct get rewritten.

What got deleted

Library:

• src/StrongTypes/Option_Old/IOptionExtensions_Old.cs — the whole user-facing extension surface (Map/FlatMap/Select/SelectMany/Where/Is/GetOr*/OrElse/Flatten/ToNullable/MapAsync/FlatMapAsync/MatchAsync/ToTry).
• src/StrongTypes/Option_Old/IOptionExtensions_CollectionsFlattening_Old.cs.
• src/StrongTypes/Option_Old/IOptionExtensions_CollectionsGetting_Old.cs — whole file was already [Obsolete].
• src/StrongTypes/Extensions_Old/IReadOnlyDictionaryExtensions_Old.cs — .Get() returning Option<V>; only consumer was DictionaryTests_Old.

Library (trimmed in place):

• IEnumerableExtensions_Options_Old keeps only Flatten<IEnumerable<Option<T>>> (used by Try_Old.Aggregate). SafeMax/SafeMin/First/Last/SingleOption on IEnumerable<T> gone — only _Old tests called them.
• IEnumerableExtensions_ReadOnlyList_Old loses SingleOption/FirstOption/LastOption. The non-Option helpers (IsMultiple, Single, First, Second…Fifth, Last, ElementAt, IndexOf) stay.
• ObjectExtensions_Generic_Old loses .As<A>() and both ToOption<A> overloads. object as A is the modern substitute for As<A>; ToOption is redundant given Maybe.
• Option_Old.True/False/Unit now use Option.Valued(...) directly instead of the deleted value.ToOption() extension.
• Two small .GetOrDefault()/.ToOption() call sites inside _Old (IEnumerableExtensions_Emptiness_Old, IEnumerableExtensions_Try_Old) now use Option.Valued(...) / option.Value directly.

Tests:

• Options_Old/ in full (22 files).
• Dictionary_Old/DictionaryTests_Old.cs (tested a deleted extension).
• Generative_Old/OptionGenerators_Old, OptionShrinkers_Old, Shrinkers_Old (Shrinkers only exposed OptionShrinkers), GeneratorHelpers_Old (SometimesEmpty + unused ToList).
• Collections_Old/First/Last/Single/FlattenTests_Old and INonEmptyEnumerable_Old/Option_CreateAndCreateFlatTests_Old.
• IEnumerableExtensionsTests_Old drops the FirstOption case; PartitionMatch and Aggregate tests stay with Option.Valued replacing .ToOption().
• NonEmptyStringEqualityTests_Old drops OptionEqualityTest; the non-Option EqualityTest stays as the only dedicated coverage of NonEmptyString's equality operators.
• OptionAssert_Old kept but slimmed to three methods — still needed by ExceptionsAggregateTests_Old, AsNonEmptyTests_Old, NonEmptyEnumerableTypeIssuesTests_Old which assert over Option<T> results from non-deleted APIs.

Numbers

• 4,700 lines deleted, 22 added.
• Unit-test count: 716 → 365, all still passing. The deleted tests exercised code that no longer exists.

Interaction with #36

Independent. The #36 maybe-type branch removed the Option-returning SafeMax/SafeMin overloads from IEnumerableExtensions_Options_Old to clear the collision with Maybe's replacements — this PR deletes the same two methods (plus much more). Whichever merges first, the other rebases cleanly onto the result.

Test plan

• Full build clean (0 warnings, 0 errors).
• Unit tests green (365/365).
• CI pipeline green (monitoring).

[KaliCZ]

Closing for now — this cleanup only makes sense once Try and Coproduct have Result and OneOf replacements. Until then Option's extensions are borderline useless but the type itself still needs to stay as Try/Coproduct surface, which leaves an awkward middle state.