Improve Wire3 error message for opaque types with custom encoders

[CharlonTank]

Problem

When users try to use an opaque type with a custom Wire3 encoder (like BiSeqDict) in their BackendModel or FrontendModel, they get a cryptic error message:

-- TOO MANY ARGS
The `w3_encode_BiSeqDict` function expects 1 argument, but it got 3 instead.

This doesn't help users understand what went wrong or how to fix it.

Solution

This PR improves the error message to provide helpful guidance:

-- WIRE3 ENCODER ISSUE -------------------------------------------------

I found an opaque type with a custom Wire3 encoder:

    BiSeqDict

The encoder `encodeBiSeqDict` exists, but I can't generate the
correct wrapper automatically because the type constructor is not exposed.

You have 3 options:

1. Expose the type constructor (simple but wasteful - sends internal structure)
   
2. Add compiler support (efficient - only sends what's needed)
   Make a PR to lamdera/compiler following the SeqDict pattern
   See: https://github.com/lamdera/compiler/blob/master/extra/Lamdera/Wire3/Encoder.hs#L170
   
3. Remove from BackendModel/FrontendModel (if not needed for Wire3)

Changes

• Modified extra/Lamdera/Wire3/Helpers.hs to detect when encode<TypeName> exists but w3_encode_<TypeName> doesn't
• Modified extra/Lamdera/Wire3/Encoder.hs to check for custom encoders in same-module case
• Shows helpful error message with 3 clear options instead of cryptic "TOO MANY ARGS"

Testing

Tested by temporarily removing BiSeqDict special-casing and compiling a project that uses BiSeqDict in BackendModel - confirmed new error message appears.

Related

This improves the developer experience for anyone working with custom opaque types in Lamdera, and will be particularly useful for the upcoming BiSeqDict, MultiSeqDict, and MultiBiSeqDict types in lamdera/containers.