Improves the TypeScript types of Collection.ts as part of the ongoing TS migration, removing hand-written as casts and @ts-expect-error suppressions in favour of properly inferred types.

find and related accessors

• Add a NSV (not-set value) type parameter to find, and type findEntry's notSetValue as [K, V].
• Drop the now-unnecessary casts in find, get, has, first and last (as V | undefined, as unknown as V, …) and remove a @ts-expect-error on notSetValue.

isSubset / isSuperset

• Replace the scattered as unknown as { … } / as unknown as CollectionImpl casts with a small hasIncludesMethod type guard.
• Rewrite isSuperset as Collection(iter).every((value) => this.includes(value)), which is behaviourally equivalent to the previous iter.isSubset(this) delegation but removes the this as unknown as Iterable<V> cast (isSubset is not overridden anywhere, so there was no polymorphism to preserve).
• The type guard checks typeof iter === 'object' before using in, so passing a primitive iterable (e.g. a string) no longer throws.

Behaviour change: isSubset/isSuperset now normalize non-Immutable inputs through Collection() (based on isCollection) instead of duck-typing an arbitrary includes/isSubset method. A plain object that is not an Immutable collection but exposes its own includes/isSubset method will no longer have that method invoked — it is treated as a regular iterable. Immutable collections and arrays are unaffected.

reduce / reduceRight

• Type the shared reduce helper generically, removing the @ts-expect-error and the as R cast at both call sites. The implementation signature now returns the honest V | R | undefined (the public overloads still return R).
• Add an Array#reduce-style overload for the no-initial case where the accumulator is a value (R = V), so the result type is inferred from the collection's values instead of requiring an explicit type argument: list.reduce((a, b) => a + b) now infers number.

Tests

• Add tstyche coverage for reduce/reduceRight (none existed before): result type with/without an initial value, the new value inference, and the reducer argument types across List, Map, Set and Collection.
• Add unit tests: isSubset against arrays/collections and a primitive string (regression for the in throw), and reduce without an initial value (single value returns the seed, empty collection returns undefined).

CHANGELOG.md updated for the user-visible behaviour and type changes.