feat: add Phase 5 document highlights provider

[j4rviscmd]

Summary

Phase 5 of the native (Rust) Language Host: tree-sitter-backed DocumentHighlightProvider, continuing Phase 0→1→2→3→4 (documentSymbol/foldingRange/hover/definition/references already merged).

Phase 5 — Document Highlights

• Message::DocumentHighlights { uri, line, column } → DocumentHighlightsResponse { highlights: [{ range, kind }] } or null
• Resolve the identifier at the cursor, then recursively walk the document AST collecting ALL identifier-like nodes whose text matches; each becomes a DocumentHighlight { range, kind } where kind is Write(2) for declaration names (is_declaration_name) and Read(1) otherwise. Text(0) is unused in v1.
• File-local only; all matches returned (no scope-aware filtering — shadowed locals highlight too)

Refactor

• Extracted point_for_position helper (shared Point resolution between hover and find_identifier_at_position)
• Unified response interfaces to use IRange

Fix

Corrected DocumentHighlightKind comment (Text=0, not Text=3 — VS Code enum, not LSP). No runtime impact (only Read=1/Write=2 are emitted).

Known limitation

File-local, name-based — no cross-file type resolution like tsserver, no scope-aware filtering. Coexists with the built-in TS provider (non-exclusive, Phase 1 pattern).

How to verify

1. `cd rust && cargo build`
2. `npm run watch` + `./scripts/code.sh`
3. Enable `coderm.languageHost.enabled` and set `coderm.languageHost.languages` (e.g. `["typescript", "tsx"]`)
4. Open a `.ts` file, place the cursor on an identifier and wait — all same-name identifiers in the file get highlighted (declaration = Write, references = Read).