feat: typed Choice<E> and ChoiceAnswer<E> with enum constants as labels

[garretpremo]

Motivation:
Implements the design accepted on #10: the Java rendering of the JS SDK's ChoiceResponse<T>. A misspelled label becomes a compile error, probabilities are keyed by the enum, and a switch over the answer is exhaustive.

Changes:

• Choice<E>: of(instructions, Dept.class) builds one option per constant; builder(Dept.class) takes option(Dept.X, …) for a subset with descriptions. of(…, String...), of(…, Map), and the existing builder infer Choice<String>. Record components are unchanged, so the wire form is identical.
• ChoiceAnswer<E>: as(Dept.class) converts a String-labelled answer to the enum, with probabilities in an EnumMap in declaration order. An unknown label throws IllegalArgumentException naming the label and the enum's constants.
• TypeSafeResponse.choice(key, Dept.class); choice(key) returns ChoiceAnswer<String> and choices() returns Map<String, ChoiceAnswer<String>>.
• TypeSafeRequest.Builder.choice(key, Dept.class, c -> …) alongside the existing String form.
• Tests: enum read with declaration-order probabilities and the unchanged String read; unknown-label rejection; enum construction, subset builder, request builder, and a wire-shape check that no enum metadata leaks.
• README "Typed choices" section; CHANGELOG entry with the recompile note.

Breaking: ChoiceAnswer gained a type parameter, which changes the erased return type of choice(). Consumers must recompile; a raw ChoiceAnswer needs <String> or var.

Fixes #10