Remove FSharp.Core compile-time dependency from Marten

[jeremydmiller]

Summary

• Completely removes the FSharp.Core PackageReference from Marten.csproj
• Replaces all direct FSharpOption<T> type references with reflection-based equivalents via new FSharpTypeHelper static class
• F# support still works when FSharp.Core is present at runtime (loaded by F# consuming projects)
• When FSharp.Core is NOT loaded, all F# checks gracefully return false/null — zero overhead for non-F# users

Closes #3981

What Changed

• New FSharpTypeHelper.cs — lazy Type.GetType("Microsoft.FSharp.Core.FSharpOption\1, FSharp.Core")` resolution
• StoreOptions.Identity.cs — RegisterFSharpOptionValueTypes() uses MakeFSharpOptionType() instead of typeof(FSharpOption<Guid>) etc.
• StoreOptions.MemberFactory.cs — type checks use FSharpTypeHelper instead of direct type references
• ConstantExpressionExtensions.cs — IsFSharpOptionType() replaces typeof(FSharpOption<>) comparison
• FSharpOptionValueTypeMember.cs — deleted (empty class, replaced by direct ValueTypeMember<,>.MakeGenericType())
• DocumentStore.cs — removed unused using Microsoft.FSharp.Control

Test plan

• 42/42 F# integration tests pass (LINQ queries, record projections, discriminated union IDs, option types)
• Compiled Marten.dll has zero FSharp.Core assembly references
• Marten builds cleanly without FSharp.Core

🤖 Generated with Claude Code