feat: add F# language support (tree-sitter + FsAutoComplete)

[jafreck]

Summary

Add F# support via tree-sitter + LSP enrichment, complementing existing C# support in the .NET ecosystem.

Tooling

Component	Tool	Notes
Tree-sitter grammar	tree-sitter-fsharp	Ionide project, actively maintained
LSP server	FsAutoComplete	Community standard, mature
SCIP indexer	None (scip-dotnet covers C#/VB but not F#)	N/A

What to implement

• Add tree-sitter-fsharp as an npm dependency
• Create src/indexer/extractors/fsharp.ts:
  • Extract let funcName args = body as function symbols
  • Extract type TypeName = ... as type symbols (discriminated unions, records, classes)
  • Extract module ModuleName as module symbols
  • Extract member this.MethodName(args) as method symbols
  • Extract function application (funcName arg1 arg2) as call refs
  • Extract pipe chains (x |> f |> g) — each piped function is a call ref
  • Extract open Module.Path as imports
• Register fsharp in SourceIndexStage EXTRACTORS map and ParserPool LANG_PACKAGES
• Add fsautocomplete to src/indexer/lsp/registry.ts: { command: 'fsautocomplete', args: ['--adaptive-lsp-server-enabled'] }
• Add file extensions .fs, .fsi, .fsx to walker
• Add test fixtures in tests/fixtures/fsharp/
• Add extractor tests

Language notes

• F# uses significant whitespace (indentation-based scoping, like Python)
• Function application is by juxtaposition (f x y), not parentheses — the extractor must recognize this pattern
• Pipe operator |> is idiomatic: data |> List.map transform |> List.filter pred — each function in the chain is a call edge
• .fsi files are signature files (like .d.ts for TypeScript) — should be indexed in declaration mode
• Discriminated unions (type Shape = Circle of float | Rectangle of float * float) are key type declarations
• Computation expressions (async { ... }, task { ... }) contain call-like constructs

Context

Complements existing C# support. F# and C# co-exist in .NET solutions and interop freely. Finance and data-heavy .NET projects increasingly use F#.