feat(ide): expand folding coverage and fix selection in preprocessor regions

[hongjr03]

Summary

Four commits improving folding and selection, driven by the review goals of correctness, cleanliness, and efficiency:

1. Expand folding coverage (crates/ide/src/folding_ranges.rs, LSP layer)

   • Fold function/task bodies (Subroutine) at file, module, and generate-block level, recursing into their statement arenas like blocks.
   • Recurse into generate blocks: instances, subroutines, assigns, and declarations inside generate now fold individually.
   • Fold runs of consecutive single-line items into one group fold: package imports (Imports), continuous assigns, and declarations.
   • Fold multi-line argument lists and concatenations/assignment patterns syntactically, like rust-analyzer's ArgList/Array folds.
   • Rewrite comment folding as a single syntax walk over token trivia; drop the memchr scan and the ineffective visited_ranges set.
   • Remove FoldingConfig from the ide API (line_fold_only is purely a client-capability concern at the LSP layer) and the never-populated collapsed_text field.

2. Selection inside preprocessor directives and inactive ifdef branches

   • Directive trivia is zero-width in the syntax tree and disabled code is absent from it entirely. Recover exact ranges from the preprocessor trace (root buffer offsets are the file's own offsets): directive keyword/name ranges for ifdef/define/include lines, and disabled_ranges for inactive branch bodies.
   • Drop the duplicate selection range that appeared for trivia-only files.

3. Fold fixture matrix + region fix

   • rust-analyzer style <fold kind> fixture harness covering every fold kind.
   • Fix a pre-existing ordering bug in RegionTreeBuilder::stage: the closing token's trivia was scanned only after open regions were force-closed, so an endregion marker attached to endmodule/EOF was lost and the region stretched to the end of the container.

4. Single-item group fold fix

   • collect_item_groups folded a lone multi-line item as a one-element group, duplicating the item fold; require at least two items per group.

Validation

• cargo test --workspace green (540+ tests, including 18 new folding fixtures and an extended selection matrix).
• cargo fmt --check clean for all touched files.

Follow-up

Opened separately: library declarations in .sv files (library-map syntax) recover into misleading multi-line declaration folds (see issue).