Adds the boolean field consumed by the upcoming Shadow DOM mount path.
Strict equality with the string "true" — anything else (absent, "false",
"1", "yes", empty) yields false, so accidental opt-in is impossible.
Tests cover the full parseConfig surface (required fields, server-url
derivation, position default) plus the new shadow attribute parsing.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
