[upstream-sync] feat(splitter): add Elm tree-sitter language support (upstream #1955)

[github-actions]

Upstream Change Summary

Type: new-feature
Difficulty: Easy
Recommendation: Adopt

Upstream added syntax-aware tree-sitter splitting for Elm (.elm files) via tree-sitter-elm 5.9.0. The PR wires up the parser to RecursiveSplitter, adds detect_code_language assertion for .elm extension, and adds round-trip tests. This follows the same pattern as the already-tracked Svelte/Vue/Julia additions (issue #165).

Upstream References

• PR #1955: feat(splitter): add Elm tree-sitter support cocoindex-io/cocoindex#1955 — add Elm tree-sitter support
• Upstream repo: https://github.com/cocoindex-io/cocoindex

Relevant Upstream Files / Areas

• rust/ops_text/ — tree-sitter language wiring and splitter implementation

Maps to recoco: crates/recoco-splitters/

Recoco Considerations

• Feature gate: Must be gated under function-split (or an Elm-specific sub-gate if granularity is desired) consistent with recoco's feature-gating architecture.
• Dependency: tree-sitter-elm 5.9.0 targets tree-sitter ^0.25.10. Verify recoco's pinned tree-sitter version is compatible before adding the dep.
• No Python code to port — the upstream change is entirely in rust/ops_text/.
• Related: This is additive and follows the same pattern as the Svelte/Vue/Julia additions tracked in issue [upstream-sync] feat(splitters): add Svelte, Vue, and Julia tree-sitter language support (upstream #1937, #1942) #165. It can be implemented in the same PR/pass as those.

Integration Notes

Straightforward adoption. Add tree-sitter-elm to crates/recoco-splitters/Cargo.toml (feature-gated), register the language in the splitter language table, and add the same tests (detect_code_language + RecursiveSplitter round-trip) as the upstream PR. Should take under an hour once the Svelte/Vue/Julia work in #165 is landed.