Protocol for encoding Elixir values to wire-safe representations.

This protocol is called during Tree.normalize/1, the single value encoding pass that converts raw Elixir values into wire-compatible forms. Widget builders (Build.put_if/3) do NOT call this protocol -- values stay as raw Elixir terms (atoms, tuples, structs) until normalization.

Key stringification (atom keys to string keys) is NOT handled here. That happens at the wire boundary in Protocol.Encode.stringify_keys/1, which runs just before serialization.

Widget authors should implement this protocol for custom value types that need special wire encoding (e.g. a struct that should become a specific map shape on the wire).

Primitive implementations

• Atom: true/false/nil pass through; other atoms become strings
• BitString: pass through
• Integer: pass through
• Float: pass through
• Tuple: converted to list with recursive encoding
• Map: values recursively encoded
• List: elements recursively encoded
• Any: raises Protocol.UndefinedError (no silent passthrough)

Struct implementations

Struct implementations live alongside their struct definitions:

• Plushie.Type.A11y -- strips nil fields, converts atom keys to strings
• Plushie.Type.Border -- encodes per-corner radius to string-keyed map
• Plushie.Type.Shadow -- encodes offset as [x, y] list
• Plushie.Type.Font -- encodes struct to wire font map (strips nils)
• Plushie.Type.Padding -- encodes struct to per-side map (strips nils)
• Plushie.Type.StyleMap -- encodes status overrides and nested structs