F# folding: the `open` and `xml_doc` arms in collect_folding are dead code

Summary

src/sharplsp/src/syntax.rs collect_folding gained F# node kinds on PR #218. Two of the new match arms can never produce a fold:

// Comments: C# `comment`, F# `block_comment` ((* *)) and `xml_doc` (///) — multi-line only.
"comment" | "block_comment" | "xml_doc"
    if node.start_position().row != node.end_position().row => { ... }
// Using directives group (C# `using_directive`, F# `open`)
"using_directive" | "open" => Some(FoldingRangeKind::Imports),

1. "open" matches a keyword token, not a declaration node

In tree-sitter-fsharp 0.3.11 (fsharp/src/node-types.json), open is "named": false — it is the bare open keyword token. The F# import declaration node is import_decl ("named": true, one long_identifier child).

The open keyword token always spans exactly one line, so start.row < end.row in the emit guard is never true. The arm is unreachable in effect.

2. "xml_doc" never spans more than one line

The grammar emits one xml_doc node per /// line, so a multi-line doc block is N single-line nodes, and the start_position().row != end_position().row guard is never satisfied. The arm is dead too.

Reproduction

Adding to the fixture in test_folding_range_on_fsharp_file (src/sharplsp/tests/e2e_modules/folding.rs):

module M
open System
(* a multi-line
   block comment *)
/// Computes the area of a shape.
/// The doc block spans two lines, so it folds.
type Shape =
    ...

textDocument/foldingRange returns 4 ranges — three region folds and exactly one comment fold (the (* *) block, lines 2–3). There is no imports fold for open System and no comment fold for the two /// lines.

The existing assertion passes only because kinds.contains(&"comment") is satisfied by the (* *) block alone.

Expected

• Consecutive /// (xml_doc) sibling nodes should merge into a single comment fold spanning the run — this is what Visual Studio / Rider / C# Dev Kit do for /// blocks, and F# is a first-class citizen (CLAUDE.md aim #2).
• Consecutive import_decl (F#) / using_directive (C#) siblings should merge into a single imports fold. Note the C# side has the same latent problem: a one-line using X; also fails the start.row < end.row guard, so FoldingRangeKind::Imports is likely never emitted for either language today.

Both need sibling-run grouping in collect_folding, not just a node-kind rename.

Notes

Coverage does not catch this: the arms share instrumented lines with live arms, so the file reports 220/224 lines covered on both Linux CI and macOS.

Found while auditing PR #218's Rust coverage gate.

Start in src/sharplsp/src/syntax.rs at collect_folding and inspect how sibling nodes are traversed and folding ranges are emitted. Reproduce the behavior with the fixture in src/sharplsp/tests/e2e_modules/folding.rs, then extend coverage for consecutive xml_doc and import_decl/using_directive nodes. Done means consecutive documentation and import declarations produce the expected comment and imports ranges.