feat: add Phase 4 references provider

[j4rviscmd]

Summary

Phase 4 of the native (Rust) Language Host: tree-sitter-backed ReferenceProvider (Find All References, Shift+F12), continuing Phase 0→1→2→3 (documentSymbol/foldingRange/hover/definition already merged).

Phase 4 — References

• Message::References { uri, line, column, include_declaration } → DefinitionResponse { locations: Location[] } or null
• Resolve the identifier at the position, then recursively walk the document AST collecting ALL identifier-like nodes whose text matches; declaration name nodes gated by include_declaration (VS Code ReferenceContext)
• File-local only; all matches returned (no scope-aware filtering)

Fix

Message::References was missing per-variant rename_all = "camelCase", so include_declaration expected snake_case while the renderer sent camelCase includeDeclaration — every references request failed to deserialize and timed out. Added the attribute (same serde trap as Phase 2).

Refactor

• Extracted find_identifier_at_position (shared prelude between definition and references)
• Unified parseDefinition/parseReferences into parseLocations
• Flattened the declaration-gating condition in collect_reference_locations

Known limitation

File-local, name-based — no cross-file type resolution like tsserver, no scope-aware filtering (shadowed locals match too). 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 a symbol, press `Shift+F12` → peek shows all references (and the declaration).

Verified end-to-end by the user (Shift+F12).