feat: add Phase 3 definition provider

[j4rviscmd]

Summary

Phase 3 of the native (Rust) Language Host: tree-sitter-backed DefinitionProvider (Go to Definition), continuing Phase 0→1→2 (documentSymbol/foldingRange/hover already merged).

Phase 3 — Definition

• Message::Definition { uri, line, column } → DefinitionResponse { locations: Location[] } or null
• Resolve the identifier at the position, then recursively walk the document AST collecting declarations whose name token matches; return all matches as Location[] (file-local only)
• All matches so VS Code shows a picker on ambiguous / overloaded / shadowed names
• Reuses Phase 2 helpers (UTF-16→UTF-8 column, declaration predicates, range_from_points); refactored hover/definition into a shared feature_json<T,F> envelope (Rust) and parseJsonObject<T> (renderer)

Known limitation

File-local, name-based resolution — no cross-file type resolution like tsserver, no scope-aware shadowing/overload filtering. The provider is non-exclusive and coexists with the built-in TS extension (Phase 1 pattern), so simple buffer-local jumps are offloaded while tsserver handles complex cases.

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 a function/variable reference, press `F12` → jumps to the declaration. Multiple same-name declarations → picker.

Verified end-to-end by the user (F12 jump).