StarterKit converter

General style: skill aspid-code-style; placement: starterkit-layout; docs: asp-xmldoc. This skill covers only what is specific to converters.

Skeleton

#nullable enable
using System;
using UnityEngine;
using Aspid.FastTools.Types;

// ReSharper disable once CheckNamespace
namespace Aspid.MVVM.StarterKit
{
    /// <summary>Clamps a number to a range.</summary>
    [Serializable]
    [TypeSelectorDisplay(
        Group = "Aspid/Numbers",
        Name = "Clamp",
        Tooltip = "Clamps a number to a range")]
    public class ClampNumberConverter : TwoWayNumberConverter
    {
        [Tooltip("Lower bound.")]
        [SerializeField] private double _min;

        [Tooltip("Upper bound.")]
        [SerializeField] private double _max = 1d;

        /// <remarks>Default: 0..1.</remarks>
        public ClampNumberConverter() { }

        /// <param name="min">Lower bound.</param>
        /// <param name="max">Upper bound.</param>
        /// <exception cref="ArgumentOutOfRangeException">Max is below min.</exception>
        public ClampNumberConverter(double min, double max)
        {
            if (max < min) throw new ArgumentOutOfRangeException(nameof(max));

            _min = min;
            _max = max;
        }

        protected override double Apply(double value) =>
            Math.Clamp(value, _min, _max);
    }
}

• The class is [Serializable] and not sealed when generic. The display name matches the class name; Group follows the family.
• [SerializeReference] fields typed IConverter<,>, ICollectionFilter<>, ICollectionOrder<>, ICanExecuteHandler, IViewFactory<>, PluralRule are not marked [TypeSelector]: StarterKitTypePickerDrawer supplies the picker by field type. [TypeSelector(...)] with arguments remains only where the candidate set must be narrowed.

Which base to use

Which types to accept

• A converter that takes a number implements the whole family int, long, float, double when integer input is meaningful (Lerp, Remap, Round, formatters). Normalized 0..1 inputs (Easing, SmoothStep, InverseLerp) and wrappers over Unity float APIs stay float plus double → double; when Mathf/AnimationCurve sits underneath, double goes through the float path and one <remarks> phrase says so.
• One vector operation lives in one class implementing IConverter<Vector2, Vector2>, <Vector3, Vector3>, <Vector4, Vector4>, named without a digit (VectorNormalizeConverter). Exception: the Vector2CombineConverter/Vector3CombineConverter bases.
• If the implementation already accepts null, declare the interface as IConverter<T?, …>; return-type nullability matches the interface.
• Public Convert(...) overloads plus explicit implementations: the explicit one delegates to the public one on the line after =>.

ConvertBack and the name

• Add ConvertBack where the reverse is unambiguous; do not force it when it is lossy or already covered by a paired converter.
• Drop "To" from the name only when a reverse interface exists: IConverter<B, A>/ITwoWayConverter<B, A> beside the forward one, or ITwoWayConverter<A, A> (DegreesRadiansConverter, ColorColor32Converter). A single ITwoWayConverter<A, B> keeps "To" (BoolToValueConverter, StringToIntConverter). ConverterAsset wrappers and caster binders are outside the rule.
• A field used only in ConvertBack: name _convertBackFallback + [UsedInModes(BindMode.TwoWay, BindMode.OneWayToSource)]. A parser field used only in Convert: [UsedInModes(BindMode.OneWay, BindMode.TwoWay, BindMode.OneTime)].

Errors and fallback

• The constructor throws on invalid arguments. Misconfigured serialized fields are detected in Convert and logged on every call through ConverterLogger; no "already logged" flags.
• ConverterFallback<T> is for runtime failures that depend on the value (unparseable string, division by zero, a root with no real result), or when runtime and configuration failures mix in one converter. Pure configuration errors (an empty required reference) get no fallback field: LogError plus a sensible return.
• The constructor takes the fallback as one optional ConverterFallback<T>? fallback = null, assigned as _fallback = fallback ?? _fallback;. The internal representation (ticks, a decimal string) never leaks into the signature.
• UseFallback/Fail calls use named arguments, one per line; the problem text comes from value.Expected("…").
• When [Min]/[Range] on a field rules the invalid value out of the Inspector, the runtime check for the same condition is removed and the constructor throws ArgumentOutOfRangeException. Attribute, exception and Tooltip agree.

Nested slots

• A parameter that duplicates another converter (a format string, a comparison value) becomes [SerializeReference] private IConverter<…>? _slot with a default implementation in the initializer (= new NumberFormatConverter("0.##")).
• Empty slot: either documented pass-through (optional) or an error on every call plus ArgumentNullException in the constructor (required).
• A convenience constructor with the plain parameter may stay, delegating into the slot.
• Do not apply to a field that is the converter's essence (the format in NumberFormatConverter).

Serialized fields

• [Tooltip] on every one, one short phrase. A behavioral caveat is repeated in the constructor <param>.
• [Min], [Range] and similar wherever the field can become invalid: indices and counts [Min(0)], fractions [Range(0f, 1f)].
• No = default!, no [FormerlySerializedAs].

Tests

• The catalogue is covered by contract tests in Tests/EditMode/Converters/Contracts/* (TypeSelectorDisplay, field coverage, picker, frozen names in ConverterRenameCompatibilityTests). Update the frozen list after a rename; make sure the contracts pass after an addition.
• Assertions and comments in English.