This commit improves memory management, reduces redundant interpreter
instantiation, and standardizes service usage patterns.
- Add `dispose()` to `ExtensionService` interface and implement it across
Dart, JS, LNReader, and Mihon services.
- Replace repeated interpreter creation in `DartExtensionService` with a
persistent `_interpreter` instance initialized once in the constructor.
- Add disposal logic for JS DOM selector and Cheerio instances to prevent
memory leaks.
- Introduce `withExtensionService()` helper to ensure services are always
disposed after use.
- Update call sites across the codebase to use `withExtensionService()`
or manual try/finally disposal.
- Improve isolate service message handling by extracting `responsePort`
earlier.
- Ensure safer defaults (e.g., returning empty lists, const lists) when
service calls fail.
