Support type-union ports (e.g. FloatValue | SequenceValue[FloatValue])

[xujustinj]

Motivation

The value/port type system currently requires each input or output field to be a single Value type (one x-value-type, or a generic with a single inner Value). There is no way to declare a port that accepts one of several value types.

This blocks clean designs elsewhere. Concretely, in #153 (more arithmetic nodes) we want a single variadic node to also accept a sequence — i.e. a param/input typed FloatValue | SequenceValue[FloatValue] — so that Add/Multiply/Min/Max subsume their Sum/Product sequence counterparts instead of shipping two nodes per operation. Without unions we're forced to duplicate (variadic node + sequence node) for every associative op.

What's needed

A union value type usable anywhere a Value type is expected:

• Schema representation — how a union is expressed in the compact x-value-type form and the expanded JSON Schema (likely an anyOf / oneOf of x-value-type entries). schema parse/check must understand it.
• Validation — a value validates against the union if it validates against any member.
• Edge type-checking — a source is assignable to a union target if it's assignable (directly or via a registered cast) to any member. possible-edges and add-edge must account for this.
• Casting interaction — define precedence when a source could cast to more than one union member (e.g. prefer exact-type match, then first castable member).
• Runtime — node run receives whichever concrete Value matched; nodes branch on the concrete type.

Open questions

• Tagged vs untagged union (do we need a discriminator, or is structural matching enough)?
• Interaction with the existing cast registry — does a union target widen the set of acceptable sources too aggressively?
• Serialization stability across versions / migrations.

Blocks

• Add more built-in arithmetic nodes #153 (arithmetic nodes) — the "variadic vs sequence vs both" decision collapses to "just variadic, with a FloatValue | SequenceValue[FloatValue] input" once unions exist.