revert: remove generic type parameter from AnyUUID/AnyInt64

[klaidliadon]

Summary

• Reverts feat: generic AnyUUID/AnyInt64 with variable prefix support #7 — removes the generic [P Prefixer] type parameter from AnyUUID and AnyInt64, returning them to simple structs with a string prefix field.
• Removes VariablePrefixer interface, AnyPrefix type, and Variant() method.

Why

The generic added complexity to a low-level package without meaningful downstream simplification. Prefix validation (matching a string prefix to an application-level enum) belongs in the application layer, not baked into the ID type via a type parameter.

Callers that need prefix validation can define helpers like:

func ParseApiKeyID(s string) (typeid.AnyUUID, ProjectMode, error) {
    id, err := typeid.ParseAnyUUID(s)
    if err != nil {
        return typeid.AnyUUID{}, 0, err
    }
    var mode ProjectMode
    if !mode.ParsePrefix(id.Prefix()) {
        return typeid.AnyUUID{}, 0, fmt.Errorf("invalid api key prefix: %q", id.Prefix())
    }
    return id, mode, nil
}

Test plan

• All 70 existing tests pass
• No breaking changes to UUID[P] or Int64[P] (compile-time-fixed prefix types unchanged)

[VojtechVitek]

LGTM. Thank you 👍