Fix incorrect flattening behavior for hidden nodes

[JustJ01]

Problem (Issue #3629)

When a node like Instance Position or Pointer Position is hidden, the flattening logic incorrectly replaces it with a default () value.

This causes:

• Type mismatches in downstream nodes (Ex ConvertNode<DVec2>)
• Graph evaluation failures
• Panics such as:
  • Cannot construct default value for hidden node
  • entered unreachable code: tried to resolve not flattened node

Expected Behavior

Hiding a node should:

• Preserve its output type
• Forward a compatible input when possible
• Never inject () unless the node’s real output type is ()

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Solution

Screen.Recording.2026-02-14.021205.mp4

Updated flatten_with_fns handling for Visible::Value(output_type).

New behavior:

1. If a compatible input exists

   • Replace the node with an identity node
   • Forward that input directly
   • Preserve type integrity

2. If no compatible input exists

   • Generate a default value using:

     TaggedValue::from_type(&output_type)

   • Insert a correctly typed constant node

3. Prevent incorrect () propagation

   • The logic no longer defaults to () unless the node’s actual output type is unit.

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Result

• Hiding Instance Position no longer crashes.
• ConvertNode<DVec2> no longer receives ().

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