fix: don't ship source files in binding packages

[MangelMaxime]

Problem

CreateFablePackageFiles includes the project's @(Compile) files (and the .fsproj) under the fable/ folder of the produced NuGet package. Its comment says this is "if FablePackageType is a library", but the target had no Condition, so it ran for binding packages too.

As a result, a pure-interop binding shipped its full F# source in fable/. Fable prefers that source over the compiled DLL, so every consumer recompiles the whole binding from source on each build. For a large binding this is very expensive (e.g. a ~194k-line binding is ~27s of Fable transform per consumer build, vs ~0.6s when consumed as a plain DLL).

Fix

Add the missing condition so the source is only packaged for library packages:

<Target Name="CreateFablePackageFiles"
    Condition="'$(FablePackageType)' == 'library'"
    ... >

A binding package now ships only its lib/*.dll, and Fable consumes it from the assembly (interfaces + [<Import>]/[<Emit>]/[<Erase>] metadata) without recompiling it.

Test

Added should not include any source file for a binding package: it packs a new binding-with-files fixture and asserts the nupkg contains no fable/ entries. The existing library tests still assert that library packages do ship their source, so both directions are covered. Full suite: 23/23 passing.

🤖 Generated with Claude Code

https://claude.ai/code/session_011rgT2YXmo7LaUFmMFNpx9k