feat(parser): add @include directive and document inline-comment behaviour

- Add `# @include path` directive to load and merge another env file inline;
  paths are relative to the including file; circular includes detected and
  reported as errors; missing files emitted as errors but parsing continues
- Document inline-comment rules: `#` ends an unquoted value (after trimming
  trailing spaces); `#` inside single- or double-quoted values is literal
- Add TestInlineComments and TestIncludeDirective test classes (17 new tests)
- Update README: new "Inline Comments" and "Including Other Files" sections,
  updated Key Features list and sample .env
- Mark ROADMAP items 28 and 29 as done
