feat(lsp): add Dart Language Server support

[amondnet]

Summary

Add Dart Language Server support to packages/lsp with system-first fallback and auto-download capability.

Background

Based on reference implementations:

• ref/multispy/src/multilspy/language_servers/dart_language_server/
• ref/serena/test/solidlsp/dart/

Key Decisions

• SDK Fallback: System-first - Check if dart is in PATH first, only download SDK if not found
• SDK Version: Dart SDK 3.7.1 (stable)
• Storage: ~/.cache/dora/dart-lsp/
• Root Detection: pubspec.yaml, pubspec.lock

Tasks

• T001 [P] Add DART_RUNTIME_DEPS constant with platform-specific SDK URLs
• T002 [P] Add getDartResourcesDir() function
• T003 Add setupDartDependencies() function (depends on T001, T002)
• T004 Add DartServer definition (depends on T003)
• T005 Export DartServer from index.ts (depends on T004)
• T006 Add DartServer to LSP_SERVERS array (depends on T004)
• T007 Add tests for DartServer (depends on T006)

Dependencies & Execution Order

Parallel Group 1 (independent):

• T001: Add DART_RUNTIME_DEPS constant
• T002: Add getDartResourcesDir() function

Sequential:

1. T003: setupDartDependencies() (needs T001, T002)
2. T004: DartServer definition (needs T003)
3. T005 + T006: Export and add to array (needs T004)
4. T007: Tests (needs T006)

Files to Modify

• packages/lsp/src/server.ts - Add DartServer definition with auto-download
• packages/lsp/src/index.ts - Export DartServer
• packages/lsp/src/__tests__/server.test.ts - Add DartServer tests